Fair Value Measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Receivable from brokers, dealers and clearing organizations:
Securities Purchased under Agreements to Resell	$ 4,852	$ 24,006
Fair Value, Measurements, Nonrecurring [Member]
Fair Value Measurement: Assets
Cash	41,120	48,671
Deposits with clearing organization	13,299	11,748
Receivable from brokers, dealers and clearing organizations:
Securities borrowed	158,454	154,090
Receivables from brokers	25,095	25,768
Securities failed to deliver	15,147	6,172
Clearing organizations	22,700	26,081
Other	3,310	2,823
Total Receivable from brokers, dealers and clearing organizations	224,706	214,934
Receivable from customers	873,815	847,386
Fair Value, Concentration of Risk, Federal Funds Sold and Securities Borrowed or Purchased under Agreements to Resell	1,235
Investments (1)	60,131	56,300
Fair Value Measurement: Liabilities
Drafts payable	29,767	39,228
Bank call loans	230,400	145,800
Payables to brokers, dealers and clearing organizations:
Securities loaned	141,654	179,875
Payable to brokers	17,976	610
Securities failed to receive	34,421	11,523
Other	45,527	29,381
Total payables to brokers, dealers and clearing organizations	239,578	221,389
Payables to customers	498,562	449,946
Securities sold under agreements to repurchase	457,809	378,084
Senior secured notes	200,042	151,782
Fair Value, Measurements, Nonrecurring [Member] | Level 1 [Member]
Fair Value Measurement: Assets
Cash	41,120	48,671
Deposits with clearing organization	13,299	11,748
Receivable from brokers, dealers and clearing organizations:
Fair Value, Concentration of Risk, Federal Funds Sold and Securities Borrowed or Purchased under Agreements to Resell	1,235
Fair Value Measurement: Liabilities
Drafts payable	29,767	39,228
Fair Value, Measurements, Nonrecurring [Member] | Level 2 [Member]
Receivable from brokers, dealers and clearing organizations:
Securities borrowed	158,454	154,090
Receivables from brokers	25,095	25,768
Securities failed to deliver	15,147	6,172
Clearing organizations	22,700	26,081
Other	3,310	2,823
Total Receivable from brokers, dealers and clearing organizations	224,706	214,934
Receivable from customers	873,815	847,386
Investments (1)	60,131	56,300
Fair Value Measurement: Liabilities
Bank call loans	230,400	145,800
Payables to brokers, dealers and clearing organizations:
Securities loaned	141,654	179,875
Payable to brokers	17,976	610
Securities failed to receive	34,421	11,523
Other	45,527	29,381
Total payables to brokers, dealers and clearing organizations	239,578	221,389
Payables to customers	498,562	449,946
Securities sold under agreements to repurchase	457,809	378,084
Senior secured notes	200,042	151,782
Fair Value, Estimate Not Practicable, Carrying (Reported) Amount [Member]
Fair Value Measurement: Assets
Cash	41,120	48,671
Deposits with clearing organization	13,299	11,748
Receivable from brokers, dealers and clearing organizations:
Securities borrowed	158,454	154,090
Receivables from brokers	25,095	25,768
Securities failed to deliver	15,147	6,172
Clearing organizations	22,700	26,081
Other	3,310	2,823
Total Receivable from brokers, dealers and clearing organizations	224,706	214,934
Receivable from customers	873,815	847,386
Fair Value, Concentration of Risk, Federal Funds Sold and Securities Borrowed or Purchased under Agreements to Resell	1,235
Investments (1)	60,131	56,300
Fair Value Measurement: Liabilities
Drafts payable	29,767	39,228
Bank call loans	230,400	145,800
Payables to brokers, dealers and clearing organizations:
Securities loaned	141,654	179,875
Payable to brokers	17,976	610
Securities failed to receive	34,421	11,523
Other	45,527	29,381
Total payables to brokers, dealers and clearing organizations	239,578	221,389
Payables to customers	498,562	449,946
Securities sold under agreements to repurchase	457,809	378,084
Senior secured notes	$ 200,000	$ 150,000