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Debt Obligations - Senior Unsecured Revolving Credit Facility (Details)
Jun. 30, 2018
USD ($)
Mar. 31, 2018
USD ($)
Aug. 22, 2017
Debt Instrument [Line Items]      
Total debt $ 826,800,000 $ 880,700,000  
Short-term debt 266,900,000 200,000,000  
Senior Unsecured Revolving Credit Facility | Revolving Credit Facilities | 2017 Credit Facility      
Debt Instrument [Line Items]      
Adjusted leverage ratio (no greater than)     3.5
Total debt 266,900,000 $ 200,000,000  
Letter of credit outstanding 16,300,000    
Amount available for future borrowings $ 716,800,000.0