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Borrowings - Partnership's Borrowings (Details) - USD ($)
Sep. 30, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Borrowing Outstanding $ 1,517,600,000 $ 1,267,600,000
Carrying Value 1,515,600,000 1,265,200,000
Senior Credit Facility Term Loan Due 5/05/2020    
Debt Instrument [Line Items]    
Borrowing Outstanding 25,000,000.0 25,000,000
Carrying Value 24,800,000 24,700,000
CLO Term Loans    
Debt Instrument [Line Items]    
Borrowing Outstanding 229,900,000 33,800,000
Carrying Value 229,900,000 33,800,000
3.875% Senior Notes Due 2/01/2023    
Debt Instrument [Line Items]    
Borrowing Outstanding 500,000,000 500,000,000
Carrying Value $ 497,500,000 497,200,000
Interest rate on senior notes (as a percent) 3.875%  
5.625% Senior Notes Due 3/30/2043    
Debt Instrument [Line Items]    
Borrowing Outstanding $ 600,000,000 600,000,000
Carrying Value $ 600,700,000 $ 600,700,000
Interest rate on senior notes (as a percent) 5.625%