Financial Instruments - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Cash and Cash Equivalents, Fair Value Disclosure	$ 28,364	$ 60,268
Cash And Securities Segregated Under Other Regulations Measure At Fair Value		11,495
Deposits With Clearing Organizations Fair Value	16,091	10,492
Securities purchased under agreements to resell	250,000	184,825
Security Owned and Sold, Not yet Purchased Fair Value, Security Owned	944,471	856,088
Investments, Fair Value Disclosure	8,049
Loans Held-for-sale, Fair Value Disclosure	15,806	75,989
Fair Value Of Securities Reverse Repurchase Agreement	250,000	184,000
Derivative Asset	12,423	4,530
Assets, Fair Value Disclosure	1,326,368	1,267,309
Security Owned and Sold, Not yet Purchased Fair Value, Security Sold, Not yet Purchased	126,092	76,314
Liabilities Related to Investment Contracts, Fair Value Disclosure	136	648
Derivative Liability	5,847	6,574
Fair Value Measurement Assets [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Cash	36,810	38,026
Deposits paid for securities borrowed	245,206	274,127
Receivables from brokers	23,086	49,803
Securities failed to deliver	51,432	9,628
Clearing organizations	22,820	27
Omnibus accounts		18,086
Other	8,773	13,202
Total Receivable from brokers, dealers and clearing organizations	351,317	364,873
Receivable from customers	951,015	868,869
Securities purchased under agreements to resell		825
Mortgage servicing rights (MSRs)	40,662	40,084
Escrow deposit	25,007	25,006
Fair Value Estimate Not Practicable Cash And Securities Segregated Under Federal And Other Regulations	19,117	24,828
Fair Value Estimate Not Practicable Deposits With Clearing Organizations	16,094	13,187
Fair Value Measurement Liabilities [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Drafts payable	32,913	48,198
Bank call loans	147,200	118,200
Deposits received for securities loaned	203,585	211,621
Securities failed to receive	37,910	5,346
Clearing organizations and other	13,937	6,348
Total payables to brokers, dealers and clearing organizations	255,432	223,315
Payables to customers	684,884	626,564
Securities sold under agreements to repurchase	816,606	757,491
Warehouse payable	14,297	54,614
Payable to third party	25,007	25,006
Senior secured notes	160,125	208,529
Carrying Value [Member] | Fair Value Measurement Assets [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Cash	36,810	38,026
Deposits paid for securities borrowed	245,206	274,127
Receivables from brokers	23,086	49,803
Securities failed to deliver	51,432	9,628
Clearing organizations	22,820	27
Omnibus accounts		18,086
Other	8,773	13,202
Total Receivable from brokers, dealers and clearing organizations	351,317	364,873
Receivable from customers	951,015	868,869
Securities purchased under agreements to resell		825
Mortgage servicing rights (MSRs)	29,115	28,879
Escrow deposit	25,007	25,006
Fair Value Estimate Not Practicable Cash And Securities Segregated Under Federal And Other Regulations	19,117	24,828
Fair Value Estimate Not Practicable Deposits With Clearing Organizations	16,094	13,187
Carrying Value [Member] | Fair Value Measurement Liabilities [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Drafts payable	32,913	48,198
Bank call loans	147,200	118,200
Deposits received for securities loaned	203,585	211,621
Securities failed to receive	37,910	5,346
Clearing organizations and other	13,937	6,348
Total payables to brokers, dealers and clearing organizations	255,432	223,315
Payables to customers	684,884	626,564
Securities sold under agreements to repurchase	816,606	757,491
Warehouse payable	14,297	54,614
Payable to third party	25,007	25,006
Senior secured notes	150,000	195,000
Fair Value [Member] | Fair Value Measurement Assets [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Cash	36,810	38,026
Deposits paid for securities borrowed	245,206	274,127
Receivables from brokers	23,086	49,803
Securities failed to deliver	51,432	9,628
Clearing organizations	22,820	27
Omnibus accounts		18,086
Other	8,773	13,202
Total Receivable from brokers, dealers and clearing organizations	351,317	364,873
Receivable from customers	951,015	868,869
Securities purchased under agreements to resell		825
Mortgage servicing rights (MSRs)	40,662	40,084
Escrow deposit	25,007	25,006
Fair Value Estimate Not Practicable Cash And Securities Segregated Under Federal And Other Regulations	19,117	24,828
Fair Value Estimate Not Practicable Deposits With Clearing Organizations	16,094	13,187
Fair Value [Member] | Fair Value Measurement Liabilities [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Drafts payable	32,913	48,198
Bank call loans	147,200	118,200
Deposits received for securities loaned	203,585	211,621
Securities failed to receive	37,910	5,346
Clearing organizations and other	13,937	6,348
Total payables to brokers, dealers and clearing organizations	255,432	223,315
Payables to customers	684,884	626,564
Securities sold under agreements to repurchase	816,606	757,491
Warehouse payable	14,297	54,614
Payable to third party	25,007	25,006
Senior secured notes	160,125	208,529
Level 1 [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Cash and Cash Equivalents, Fair Value Disclosure	28,364	60,268
Cash And Securities Segregated Under Other Regulations Measure At Fair Value		11,495
Deposits With Clearing Organizations Fair Value	16,091	10,492
Security Owned and Sold, Not yet Purchased Fair Value, Security Owned	672,033	629,243
Assets, Fair Value Disclosure	716,896	722,273
Security Owned and Sold, Not yet Purchased Fair Value, Security Sold, Not yet Purchased	117,835	57,414
Liabilities Related to Investment Contracts, Fair Value Disclosure	136	648
Derivative Liability	188	186
Level 1 [Member] | Fair Value Measurement Assets [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Cash	36,810	38,026
Securities purchased under agreements to resell		825
Escrow deposit	25,007	25,006
Fair Value Estimate Not Practicable Cash And Securities Segregated Under Federal And Other Regulations	19,117	24,828
Fair Value Estimate Not Practicable Deposits With Clearing Organizations	16,094	13,187
Level 1 [Member] | Fair Value Measurement Liabilities [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Drafts payable	32,913	48,198
Bank call loans	147,200	118,200
Payable to third party	25,007	25,006
Level 2 [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Security Owned and Sold, Not yet Purchased Fair Value, Security Owned	179,838	141,485
Loans Held-for-sale, Fair Value Disclosure	15,806	75,989
Fair Value Of Securities Reverse Repurchase Agreement	250,000	184,000
Derivative Asset	1,895	2,155
Assets, Fair Value Disclosure	497,565	451,355
Security Owned and Sold, Not yet Purchased Fair Value, Security Sold, Not yet Purchased	8,257	18,900
Derivative Liability	312	135
Level 2 [Member] | Fair Value Measurement Assets [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Deposits paid for securities borrowed	245,206	274,127
Receivables from brokers	23,086	49,803
Securities failed to deliver	51,432	9,628
Clearing organizations	22,820	27
Omnibus accounts		18,086
Other	8,773	13,202
Total Receivable from brokers, dealers and clearing organizations	351,317	364,873
Receivable from customers	951,015	868,869
Level 2 [Member] | Fair Value Measurement Liabilities [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Deposits received for securities loaned	203,585	211,621
Securities failed to receive	37,910	5,346
Clearing organizations and other	13,937	6,348
Total payables to brokers, dealers and clearing organizations	255,432	223,315
Payables to customers	684,884	626,564
Securities sold under agreements to repurchase	816,606	757,491
Warehouse payable	14,297	54,614
Senior secured notes	160,125	208,529
Level 3 [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Security Owned and Sold, Not yet Purchased Fair Value, Security Owned	92,600	85,360
Derivative Asset	10,528	2,375
Assets, Fair Value Disclosure	111,907	93,681
Derivative Liability	5,347	6,253
Level 3 [Member] | Fair Value Measurement Assets [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Mortgage servicing rights (MSRs)	$ 40,662	$ 40,084