Borrowings - Broker-Dealer Credit Facilities (Details) $ in Thousands		3 Months Ended		9 Months Ended
	Apr. 15, 2015	Sep. 30, 2016 USD ($) item	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) item	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)	Apr. 24, 2015 USD ($)
Long-Term Borrowings
Interest expense		$ 7,393	$ 7,205	$ 21,569	$ 22,066
Broker-Dealer Credit Facilities
Long-Term Borrowings
Number of secured credit facilities | item		2		2
Broker-Dealer Credit Facility on an uncommitted basis
Long-Term Borrowings
Number of secured credit facilities | item		1		1
Maximum borrowing capacity		$ 125,000		$ 125,000
Interest rate (as a percent)		1.29%		1.29%		1.25%
Interest expense		$ 300	$ 300	$ 800	$ 600
Outstanding principal balance		$ 17,600		$ 17,600		$ 45,000
Broker-Dealer Credit Facility on committed basis
Long-Term Borrowings
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%