Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Derivative Financial Instruments (Details)

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Summary of Significant Accounting Policies - Derivative Financial Instruments (Details) - USD ($)
 1 Months Ended 3 Months Ended 12 Months Ended
Sep. 30, 2019
Dec. 31, 2018
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Sep. 25, 2019
Jul. 09, 2010
Derivative [Line Items]              
Accumulated other comprehensive income (loss) on derivative instruments, tax $ (700,000)   $ (700,000) $ (700,000)      
Other     500,000 (6,700,000) $ 200,000    
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax (13,600,000)   (12,100,000) (13,600,000)      
Carrying Value              
Derivative [Line Items]              
Derivative Asset 2,500,000   2,600,000 2,500,000      
Derivative Liability 2,600,000   1,600,000 2,600,000      
Cash Flow Hedging | Foreign Exchange Forward [Member]              
Derivative [Line Items]              
Derivative Instruments and Hedges, Assets 128,900,000   232,800,000 128,900,000      
Cash Flow Hedging | Interest Rate Swap [Member]              
Derivative [Line Items]              
Derivative Instruments and Hedges, Assets $ 50,000,000.0     50,000,000.0      
Derivative Instruments and Hedges, Liabilities     20,200,000        
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax     $ 13,800,000        
Maximum              
Derivative [Line Items]              
Length of Time Hedged in Currency Exchange Rates     24 months        
Senior unsecured notes              
Derivative [Line Items]              
Debt issued           $ 375,000,000.0  
Term of debt instrument 7 years            
$150.0 senior unsecured notes              
Derivative [Line Items]              
Debt issued             $ 150,000,000.0
Term of debt instrument   10 years          
Reclassification out of Accumulated Other Comprehensive Income              
Derivative [Line Items]              
Other     $ 9,100,000 8,200,000 1,500,000    
Interest expense, interest rate swap amortization over next twelve months     2,000,000.0        
Reclassification out of Accumulated Other Comprehensive Income | Interest Rate Swap [Member]              
Derivative [Line Items]              
Other     6,400,000        
Net Unrealized Gain (Loss) on Derivative Instruments | Reclassification out of Accumulated Other Comprehensive Income              
Derivative [Line Items]              
Other     $ 2,000,000.0 $ 6,500,000 $ 2,300,000