Financial Instruments - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Cash and Cash Equivalents, Fair Value Disclosure	$ 33,364	$ 60,268
Cash And Securities Segregated Under Other Regulations Measure At Fair Value		11,495
Deposits With Clearing Organizations Fair Value	16,595	10,492
Securities purchased under agreements to resell	250,000	184,825
Security Owned and Sold, Not yet Purchased Fair Value, Security Owned	806,772	856,088
Investments, Fair Value Disclosure	7,718
Loans Held-for-sale, Fair Value Disclosure	20,095	75,989
Fair Value Of Securities Reverse Repurchase Agreement	250,000	184,000
Derivative Asset	7,285	4,530
Assets, Fair Value Disclosure	1,192,229	1,267,309
Security Owned and Sold, Not yet Purchased Fair Value, Security Sold, Not yet Purchased	128,262	76,314
Liabilities Related to Investment Contracts, Fair Value Disclosure		648
Derivative Liability	4,542	6,574
Fair Value Measurement Assets [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Cash	44,578	38,026
Deposits paid for securities borrowed	267,643	274,127
Receivables from brokers	52,616	49,803
Securities failed to deliver	24,476	9,628
Clearing organizations	21,705	27
Omnibus accounts		18,086
Other	14,503	13,202
Total Receivable from brokers, dealers and clearing organizations	380,943	364,873
Receivable from customers	883,133	868,869
Securities purchased under agreements to resell		825
Mortgage servicing rights (MSRs)	41,768	40,084
Escrow deposit	25,014	25,006
Fair Value Estimate Not Practicable Cash And Securities Segregated Under Federal And Other Regulations	19,106	24,828
Fair Value Estimate Not Practicable Deposits With Clearing Organizations	15,016	13,187
Fair Value Measurement Liabilities [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Drafts payable	24,316	48,198
Bank call loans	135,500	118,200
Deposits received for securities loaned	164,095	211,621
Securities failed to receive	32,297	5,346
Clearing organizations and other	8,558	6,348
Total payables to brokers, dealers and clearing organizations	204,950	223,315
Payables to customers	689,363	626,564
Securities sold under agreements to repurchase	688,310	757,491
Warehouse payable	17,540	54,614
Payable to third party	25,014	25,006
Senior secured notes	158,063	208,529
Carrying Value [Member] | Fair Value Measurement Assets [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Cash	44,578	38,026
Deposits paid for securities borrowed	267,643	274,127
Receivables from brokers	52,616	49,803
Securities failed to deliver	24,476	9,628
Clearing organizations	21,705	27
Omnibus accounts		18,086
Other	14,503	13,202
Total Receivable from brokers, dealers and clearing organizations	380,943	364,873
Receivable from customers	883,133	868,869
Securities purchased under agreements to resell		825
Mortgage servicing rights (MSRs)	29,496	28,879
Escrow deposit	25,014	25,006
Fair Value Estimate Not Practicable Cash And Securities Segregated Under Federal And Other Regulations	19,106	24,828
Fair Value Estimate Not Practicable Deposits With Clearing Organizations	15,016	13,187
Carrying Value [Member] | Fair Value Measurement Liabilities [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Drafts payable	24,316	48,198
Bank call loans	135,500	118,200
Deposits received for securities loaned	164,095	211,621
Securities failed to receive	32,297	5,346
Clearing organizations and other	8,558	6,348
Total payables to brokers, dealers and clearing organizations	204,950	223,315
Payables to customers	689,363	626,564
Securities sold under agreements to repurchase	688,310	757,491
Warehouse payable	17,540	54,614
Payable to third party	25,014	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	44,578	38,026
Deposits paid for securities borrowed	267,643	274,127
Receivables from brokers	52,616	49,803
Securities failed to deliver	24,476	9,628
Clearing organizations	21,705	27
Omnibus accounts		18,086
Other	14,503	13,202
Total Receivable from brokers, dealers and clearing organizations	380,943	364,873
Receivable from customers	883,133	868,869
Securities purchased under agreements to resell		825
Mortgage servicing rights (MSRs)	41,768	40,084
Escrow deposit	25,014	25,006
Fair Value Estimate Not Practicable Cash And Securities Segregated Under Federal And Other Regulations	19,106	24,828
Fair Value Estimate Not Practicable Deposits With Clearing Organizations	15,016	13,187
Fair Value [Member] | Fair Value Measurement Liabilities [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Drafts payable	24,316	48,198
Bank call loans	135,500	118,200
Deposits received for securities loaned	164,095	211,621
Securities failed to receive	32,297	5,346
Clearing organizations and other	8,558	6,348
Total payables to brokers, dealers and clearing organizations	204,950	223,315
Payables to customers	689,363	626,564
Securities sold under agreements to repurchase	688,310	757,491
Warehouse payable	17,540	54,614
Payable to third party	25,014	25,006
Senior secured notes	158,063	208,529
Level 1 [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Cash and Cash Equivalents, Fair Value Disclosure	33,364	60,268
Cash And Securities Segregated Under Other Regulations Measure At Fair Value		11,495
Deposits With Clearing Organizations Fair Value	16,595	10,492
Security Owned and Sold, Not yet Purchased Fair Value, Security Owned	555,189	629,243
Assets, Fair Value Disclosure	605,246	722,273
Security Owned and Sold, Not yet Purchased Fair Value, Security Sold, Not yet Purchased	116,944	57,414
Liabilities Related to Investment Contracts, Fair Value Disclosure		648
Derivative Liability	284	248
Level 1 [Member] | Fair Value Measurement Assets [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Cash	44,578	38,026
Securities purchased under agreements to resell		825
Escrow deposit	25,014	25,006
Fair Value Estimate Not Practicable Cash And Securities Segregated Under Federal And Other Regulations	19,106	24,828
Fair Value Estimate Not Practicable Deposits With Clearing Organizations	15,016	13,187
Level 1 [Member] | Fair Value Measurement Liabilities [Member]
Financial Instruments Not Measured at Fair Value on Recurring Basis [Line items]
Drafts payable	24,316	48,198
Bank call loans	135,500	118,200
Payable to third party	25,014	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	164,131	141,485
Loans Held-for-sale, Fair Value Disclosure	20,095	75,989
Fair Value Of Securities Reverse Repurchase Agreement	250,000	184,000
Derivative Asset	1,774	2,155
Assets, Fair Value Disclosure	485,597	451,355
Security Owned and Sold, Not yet Purchased Fair Value, Security Sold, Not yet Purchased	11,318	18,900
Derivative Liability	33	73
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	267,643	274,127
Receivables from brokers	52,616	49,803
Securities failed to deliver	24,476	9,628
Clearing organizations	21,705	27
Omnibus accounts		18,086
Other	14,503	13,202
Total Receivable from brokers, dealers and clearing organizations	380,943	364,873
Receivable from customers	883,133	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	164,095	211,621
Securities failed to receive	32,297	5,346
Clearing organizations and other	8,558	6,348
Total payables to brokers, dealers and clearing organizations	204,950	223,315
Payables to customers	689,363	626,564
Securities sold under agreements to repurchase	688,310	757,491
Warehouse payable	17,540	54,614
Senior secured notes	158,063	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	87,452	85,360
Derivative Asset	5,511	2,375
Assets, Fair Value Disclosure	101,386	93,681
Derivative Liability	4,225	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)	$ 41,768	$ 40,084