Borrowings - Broker-Dealer Credit Facilities (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) subsidiary item	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) subsidiary item	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Long-Term Borrowings
Borrowing Outstanding	$ 1,481,108		$ 1,481,108
Borrowing Outstanding, net	$ 1,434,629		$ 1,434,629		$ 564,957
Number of broker-dealer subsidiaries | item	3		3
Unused Available Capacity	$ 0		$ 0		100,000
Interest expense	24,593	$ 7,393	40,141	$ 21,569
Broker-Dealer Credit Facilities
Long-Term Borrowings
Financing Available	125,000		125,000		200,000
Borrowing Outstanding	$ 15,000		$ 15,000		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	$ 15,000		$ 15,000		$ 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	$ 400	300	$ 1,400	800
Broker-Dealer Credit Facility on committed basis
Long-Term Borrowings
Financing Available					$ 75,000
Number of broker-dealer subsidiaries the committed facility is available to | subsidiary	1		1
Maximum borrowing capacity	$ 75,000		$ 75,000
Short-Term Credit Facilities
Long-Term Borrowings
Financing Available	543,000		543,000		493,000
Borrowing Outstanding	159,128		159,128		309,086
Interest expense	1,400	$ 1,800	4,800	$ 5,000
Total Short-term credit facilities
Long-Term Borrowings
Financing Available	543,000		543,000		493,000
Borrowing Outstanding	159,128		159,128		309,086
Revolving credit facility
Long-Term Borrowings
Unused Available Capacity					100,000
Senior Secured Second Lien Notes
Long-Term Borrowings
Borrowing Outstanding	500,000		500,000
Borrowing Outstanding, net	475,485		475,485
SBI Bonds
Long-Term Borrowings
Borrowing Outstanding	31,108		31,108		29,925
Borrowing Outstanding, net	$ 31,055		$ 31,055		$ 29,853