Borrowings - Broker-Dealer Credit Facilities (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) subsidiary	Dec. 31, 2016 USD ($) subsidiary	Dec. 31, 2015 USD ($)	Dec. 31, 2017 item	Dec. 31, 2017	Dec. 31, 2017 USD ($)	Nov. 03, 2017 USD ($) item
Long-Term Borrowings
Borrowing Outstanding						$ 1,431,059,000
Borrowing Outstanding, net		$ 564,957,000				1,388,548,000
Number of broker-dealer subsidiaries	3	3		3
Interest expense	$ 64,107,000	$ 28,327,000	$ 29,254,000
LIBOR rate
Long-Term Borrowings
Interest rate margin (as a percent)	1.25%
Broker-Dealer Credit Facilities
Long-Term Borrowings
Financing Available		200,000,000				650,000,000	$ 500.0
Number of borrowing bases | item							2
Borrowing Outstanding		25,000,000				32,000,000
Deferred Debt Issuance Cost						(4,117,000)
Borrowing Outstanding, net		25,000,000				27,883,000
Number of secured credit facilities | item				2
Number of broker-dealer subsidiaries the committed facility is available to | item				2
Maximum borrowing capacity						25,000,000
Commitment fee (as a percent)	0.50%
Interest expense	$ 1,719,000	1,232,000	918,000
Broker-Dealer Credit Facilities | LIBOR rate
Long-Term Borrowings
Interest rate margin (as a percent)	2.50%
Broker-Dealer Credit Facility on an uncommitted basis
Long-Term Borrowings
Financing Available		125,000,000				150,000,000
Borrowing Outstanding		25,000,000				25,000,000
Borrowing Outstanding, net		$ 25,000,000				25,000,000
Number of secured credit facilities | item				1
Interest rate (as a percent)		1.66%			2.42%
Interest expense	$ 1,667,000	$ 1,191,000	903,000
Broker-Dealer Credit Facility on committed basis
Long-Term Borrowings
Financing Available		75,000,000
Number of secured credit facilities | item				1
Interest expense	33,000	41,000	15,000
Short-Term Credit Facilities
Long-Term Borrowings
Financing Available		493,000,000				543,000,000
Borrowing Outstanding		309,086,000				205,677,000
Interest expense	6,600,000	6,300,000	$ 5,500,000
Revolving credit facility
Long-Term Borrowings
Financing Available						500,000,000
Borrowing Outstanding						7,000,000
Deferred Debt Issuance Cost						(4,117,000)
Borrowing Outstanding, net						2,883,000
Interest rate (as a percent)					2.81%
Interest expense	$ 19,000
SBI Bonds
Long-Term Borrowings
Borrowing Outstanding		29,925,000				31,059,000
Deferred Debt Issuance Cost		(71,000)				(47,000)
Borrowing Outstanding, net		$ 29,854,000				31,012,000
Maximum | Broker-Dealer Credit Facilities
Long-Term Borrowings
Maximum borrowing capacity						100,000,000
Minimum | Broker-Dealer Credit Facilities
Long-Term Borrowings
Maximum borrowing capacity						$ 40,000,000