Borrowings - Broker-Dealer Credit Facilities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Oct. 27, 2016 USD ($)	Jul. 18, 2016 USD ($) item	Apr. 15, 2015 USD ($)	Nov. 08, 2013 USD ($)
Long-Term Borrowings
Borrowing Outstanding	$ 569,925
Borrowing Outstanding	$ 564,957	$ 493,589
Number of broker-dealer subsidiaries | item	2
Interest expense	$ 28,327	29,254	$ 30,894
Broker-Dealer Credit Facilities
Long-Term Borrowings
Financing Available	200,000	200,000
Borrowing Outstanding	$ 25,000	45,000
Number of secured credit facilities | item	2
Broker-Dealer Credit Facility on an uncommitted basis
Long-Term Borrowings
Financing Available	$ 125,000	125,000
Borrowing Outstanding	25,000	$ 45,000
Maximum borrowing capacity	$ 125,000
Interest rate (as a percent)	1.66%	1.25%
Interest expense	$ 1,200	$ 900	0
Broker-Dealer Credit Facility on committed basis
Long-Term Borrowings
Financing Available	$ 75,000	75,000
Number of broker-dealer subsidiaries | item					1
Maximum borrowing capacity					$ 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	478,000
Borrowing Outstanding	309,086	219,089
Interest expense	6,300	5,500	$ 3,300
Total Short-term credit facilities
Long-Term Borrowings
Financing Available	493,000	478,000
Borrowing Outstanding	309,086	219,089
Senior Secured Credit Facility
Long-Term Borrowings
Borrowing Outstanding	540,000	499,800					$ 510,000
Borrowing Outstanding	535,104	493,589
Revolving credit facility
Long-Term Borrowings
Financing Available	100,000	100,000
Maximum borrowing capacity				$ 100,000		$ 100,000
Total Long-term borrowings
Long-Term Borrowings
Financing Available	100,000	100,000
Borrowing Outstanding	564,957	$ 493,589
SBI Bonds
Long-Term Borrowings
Borrowing Outstanding	29,925
Borrowing Outstanding	$ 29,853