Borrowings - Broker-Dealer Credit Facilities (Details) $ in Thousands			3 Months Ended		6 Months Ended
	Jul. 18, 2016	Apr. 15, 2015	Jun. 30, 2017 USD ($) subsidiary item	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) subsidiary item	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Long-Term Borrowings
Borrowing Outstanding			$ 1,681,142		$ 1,681,142
Borrowing Outstanding, net			$ 1,628,323		$ 1,628,323		$ 564,957
Number of broker-dealer subsidiaries | item			2		2
Unused Available Capacity			$ 100,000		$ 100,000		100,000
Interest expense			8,720	$ 7,075	15,548	$ 14,176
Broker-Dealer Credit Facilities
Long-Term Borrowings
Financing Available			200,000		200,000		200,000
Borrowing Outstanding			$ 13,000		$ 13,000		25,000
Number of secured credit facilities | item			2		2
Broker-Dealer Credit Facility on an uncommitted basis
Long-Term Borrowings
Financing Available			$ 125,000		$ 125,000		125,000
Borrowing Outstanding			$ 13,000		$ 13,000		$ 25,000
Number of secured credit facilities | item			1		1
Maximum borrowing capacity			$ 125,000		$ 125,000
Interest rate (as a percent)			2.16%		2.16%		1.66%
Interest expense			$ 500	300	$ 900	500
Broker-Dealer Credit Facility on committed basis
Long-Term Borrowings
Financing Available			$ 75,000		$ 75,000		$ 75,000
Number of broker-dealer subsidiaries the committed facility is available to | subsidiary			1		1
Maximum borrowing capacity			$ 75,000		$ 75,000
Credit facility term	364 days
Broker-Dealer Credit Facility on committed basis | LIBOR rate
Long-Term Borrowings
Interest rate margin (as a percent)					1.25%
Broker-Dealer Credit Facility on committed basis | Base rate
Long-Term Borrowings
Interest rate margin (as a percent)		1.25%
Short-Term Credit Facilities
Long-Term Borrowings
Financing Available			493,000		$ 493,000		493,000
Borrowing Outstanding			160,345		160,345		309,086
Interest expense			1,700	$ 1,400	3,400	$ 3,100
Total Short-term credit facilities
Long-Term Borrowings
Financing Available			493,000		493,000		493,000
Borrowing Outstanding			160,345		160,345		309,086
Revolving credit facility
Long-Term Borrowings
Unused Available Capacity			100,000		100,000		100,000
Senior Secured Second Lien Notes
Long-Term Borrowings
Borrowing Outstanding			500,000		500,000
Borrowing Outstanding, net			474,639		474,639
SBI Bonds
Long-Term Borrowings
Borrowing Outstanding			31,142		31,142		29,925
Borrowing Outstanding, net			$ 31,082		$ 31,082		$ 29,853