Fair Value Measurements - Changes in Level 3 Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016
Interest Rate Lock Commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance		$ 0	[1]	$ 9,161
Total Realized and Unrealized Gains (Losses), Assets	[2],[3]			4,345
Sales and Settlements	[4],[5]			(13,506)
Assets Ending Balance	[1]			0
Liabilities Beginning Balance		0		923
Total Realized and Unrealized Gains (Loss), Liabilities				923
Liabilities Ending Balance				0
Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance	[1]	158	[5]	157
Total Realized and Unrealized Gains (Losses), Assets		11	[4],[5]	1	[1],[2],[3]
Purchases and Issuances	[5]	0
Sales and Settlements	[5]	0
Transfers In (Out)	[5]	0
Assets Ending Balance	[5]	169		158	[1]
Auction Rate Securities Purchase Commitment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance	[5]	849		0
Total Realized and Unrealized Gains (Losses), Assets	[4],[5]	(849)		849
Purchases and Issuances	[5]	0
Sales and Settlements	[5]	0
Transfers In (Out)	[5]	0
Assets Ending Balance	[5]	0		849
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	[5]	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases, (Sales), Issuances, (Settlements)	[5]	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers Into Level 3	[5]	0
Auction rate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance		84,926	[6],[7]	86,802
Total Realized and Unrealized Gains (Losses), Assets		1,177	[4],[6],[7]	1,974	[2],[3]
Purchases and Issuances		27,225	[6],[7]	13,775	[8]
Sales and Settlements		(1,475)	[6],[7]	(17,625)
Transfers In (Out)		(24,455)
Assets Ending Balance	[6],[7]	87,398		84,926
Auction Rate Securities Purchase Commitment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities Beginning Balance	[9]	645		1,369
Total Realized and Unrealized Gains (Loss), Liabilities	[5]	637		724
Liabilities Ending Balance		8		645	[9]
Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Beginning Balance		44		81
Total Realized and Unrealized Gains (Losses), Assets		(9)	[4]	25	[2],[3]
Purchases and Issuances		0
Sales and Settlements		0		(62)
Transfers In (Out)		0
Assets Ending Balance		35		$ 44
Fair Value, Inputs, Level 2 [Member] | Auction rate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers In (Out)		$ 24,500

[1] (7)Sales and settlements for the ARS purchase commitments represent additional purchase commitments made during the period for regulatory and legal ARS settlements and awards.

[2] (3)Represents the difference in principal and fair value for auction rate securities purchase commitments outstanding at the end of the period.

[3] 6)Purchases and issuances in connection with ARS purchase commitments represent instances in which the Company purchased ARS securities from clients during the period pursuant to regulatory and legal settlements and awards that satisfy the outstanding commitment to purchase obligation. This also includes instances where the ARS issuer has redeemed ARS where the Company had an outstanding purchase commitment prior to the Company purchasing those ARS.

[4] )Unrealized gains (losses) are attributable to assets or liabilities that are still held at the reporting date.(Expressed in thousands) Level 3 Assets and Liabilities For the Year Ended December 31, 2016 BeginningBalance Total Realizedand UnrealizedGains (4)(5) Purchasesand Issuances Sales and Settlements TransfersIn (Out) EndingBalance Assets Municipal obligations$81 $25 $— $(62) $— $44Auction rate securities (1)(6)(7) 86,802 1,974 13,775 (17,625) — 84,926Interest rate lock commitments (2)9,161 4,345 — (13,506) — —Investments157 1 — — — 158ARS purchase commitments (3)— 849 — — — 849Liabilities Interest rate lock commitments (2)923 923 — — — —ARS purchase commitments (3)1,369 724 — — — 645

[5] 3)Included in principal transactions in the consolidated statement of operations, except for gains from investments which are included in other income in the consolidated statement of operations.

[6] 1)Represents auction rate preferred securities, municipal auction rate securities and student loan auction rate securities that failed in the auction rate market.

[7] nterest rate lock commitment assets and liabilities are recorded upon the commitment to originate a loan with a borrower and sell the loan to an investor. The commitment assets and liabilities are recognized at fair value, which reflects the fair value of the contractual loan origination-related fees and sale premiums, net of co-broker fees, and the estimated fair value of the expected net future cash flows associated with the servicing of the loan.

[8] 5)Unrealized gains are attributable to assets or liabilities that are still held at the reporting date.

[9] 4)Included in principal transactions in the consolidated statement of operations, except for gains from investments which are included in other income in the consolidated statement of operations.