Borrowings - Additional Information (Details) - USD ($)		1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Oct. 03, 2013	Mar. 31, 2013	Jan. 31, 2013	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Jan. 01, 2016	Mar. 31, 2014
Debt Instrument [Line Items]
Borrowing Outstanding				$ 1,279,700,000		$ 1,279,700,000		$ 1,138,700,000
Interest expense				15,600,000	$ 14,500,000	46,300,000	$ 43,600,000
Amount of promissory note for strategic investment contingent consideration				120,000,000		$ 120,000,000
Basis spread of variable rate promissory note (as a percent)						2.50%
Fair value of collateralized loan obligation assets				3,600,000,000		$ 3,600,000,000		18,600,000,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Borrowing capacity				750,000,000.0		$ 750,000,000.0
3.875% Senior Notes Due 2/01/2023
Debt Instrument [Line Items]
Borrowing Outstanding			$ 500,000,000.0
Loan maturity date						Feb. 01, 2023
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				526,200,000		$ 526,200,000		515,200,000
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]
Borrowing Outstanding		$ 400,000,000.0								$ 200,000,000.0
Loan maturity date						Mar. 30, 2043
Interest rate on senior notes (as a percent)		5.625%								5.625%
Senior notes percentage of par value		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	$ 8,400,000	$ 25,300,000	$ 25,300,000
Fair value of notes payable				652,500,000		652,500,000		646,600,000
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%
October 2013 CLO Term Loan
Debt Instrument [Line Items]
Borrowing Outstanding	$ 12,600,000			14,100,000		$ 14,100,000		$ 13,700,000
Loan maturity date						Sep. 28, 2018		Sep. 28, 2018
Interest rate spread on term loan to fund investment in a CLO (as a percent)	1.75%
Senior Credit Facility Term Loan Due 5/05/2020
Debt Instrument [Line Items]
Borrowing Outstanding				$ 25,000,000		$ 25,000,000		$ 25,000,000
Loan maturity date						May 05, 2020		May 05, 2020
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						1.66%
Promissory Note Due 1/01/2022
Debt Instrument [Line Items]
Borrowing Outstanding				$ 120,000,000		$ 120,000,000		$ 0
Loan maturity date						Jan. 01, 2022		Jan. 01, 2022
Actual percentage at period end						3.13%
Amount of promissory note for strategic investment contingent consideration									$ 120,000,000.0
Basis spread of variable rate promissory note (as a percent)						2.50%