Borrowings - Additional Information (Details) € in Millions		1 Months Ended		3 Months Ended			9 Months Ended
	Oct. 03, 2013 EUR (€)	Mar. 31, 2013 USD ($)	Jan. 31, 2013 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Mar. 31, 2013 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Mar. 31, 2014 USD ($)
Debt Instrument [Line Items]
Maximum borrowing capacity in term loan				$ 25,000,000.0			$ 25,000,000.0
Borrowing capacity				750,000,000.0			750,000,000.0
Maximum borrowing				1,139,100,000			1,139,100,000		$ 1,140,200,000
Interest expense				14,500,000	$ 14,400,000		43,600,000	$ 41,100,000
Accumulated other comprehensive loss related to cash flow hedges		$ 8,800,000				$ 8,800,000
Fair value of collateralized loan obligation assets				18,000,000,000			$ 18,000,000,000		17,600,000,000
Interest Rate Swap | Not designated as a hedging instrument
Debt Instrument [Line Items]
Loan maturity date							Sep. 30, 2016
Notional amount				293,750,000.00			$ 293,750,000.00
Interest Rate Swaps Second | Not designated as a hedging instrument
Debt Instrument [Line Items]
Notional amount				268,750,000.00			$ 268,750,000.00
Derivative maturity date							Sep. 30, 2016
3.875% Senior Notes Due 2/01/2023
Debt Instrument [Line Items]
Loan maturity date							Feb. 01, 2023
Senior notes principal amount			$ 500,000,000.0
Interest rate on senior notes (as a percent)			3.875%
Senior notes percentage of par value			99.966%
Date of first required interest payment							Aug. 01, 2013
Senior notes redemption terms percentage to principal amount			100.00%
Interest expense				5,000,000	5,000,000		$ 14,900,000	14,900,000
Fair value of notes payable				516,200,000			$ 516,200,000		521,200,000
3.875% Senior Notes Due 2/01/2023 | Treasury Rate
Debt Instrument [Line Items]
Interest rate spread on term loan to fund investment in a CLO (as a percent)			30.00%
5.625% Senior Notes Due 3/30/2043
Debt Instrument [Line Items]
Loan maturity date							Mar. 30, 2043
Senior notes principal amount		$ 400,000,000.0				$ 400,000,000.0				$ 200,000,000.0
Interest rate on senior notes (as a percent)		5.625%				5.625%				5.625%
Senior notes percentage of par value		99.583%				99.583%				104.315%
Date of first required interest payment							Sep. 30, 2013
Senior notes redemption terms percentage to principal amount		100.00%
Interest expense					$ 8,400,000		$ 25,300,000	$ 23,100,000
Fair value of notes payable				633,900,000			$ 633,900,000		707,600,000
5.625% Senior Notes Due 3/30/2043 | Treasury Rate
Debt Instrument [Line Items]
Interest rate spread on term loan to fund investment in a CLO (as a percent)		40.00%
Senior Credit Facility Term Loan Due 5/05/2020
Debt Instrument [Line Items]
Loan maturity date							May 05, 2020
Maximum borrowing				$ 25,000,000			$ 25,000,000		$ 25,000,000
Term Loan Principal and Interest Due in Quarterly Installment
Debt Instrument [Line Items]
Loan maturity date							Aug. 09, 2018
Revolving Credit Facility
Debt Instrument [Line Items]
Maximum percentage of applicable margin in addition to base rate				0.75%			0.75%
Maximum percentage of applicable margin				1.75%			1.75%
Actual percentage at period end							1.33%
Term loan principal prepaid						$ 475,000,000
New Term Loan
Debt Instrument [Line Items]
Maximum borrowing	€ 12.6			$ 14,100,000			$ 14,100,000
Interest rate spread on term loan to fund investment in a CLO (as a percent)	1.75%
Actual percentage at period end for term loan that funded investment in CLO							1.75%
Maturity period	5 years