Fair Value Measurements - Estimated Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financial assets:
Held to maturity securities	$ 359,542	$ 388,930
Broker-dealer and clearing organization receivables	1,222,627	1,780,280
Financial liabilities:
Broker-dealer and clearing organization payables	1,158,628	1,605,518
Carrying Amount
Financial assets:
Cash and cash equivalents	1,655,877	434,020
Assets segregated for regulatory purposes	194,626	157,436
Securities purchased under agreements to resell	161,457	59,031
Held to maturity securities	343,198	386,326
Loans held for sale	143,256	170,648
Loans held for investment, net	7,693,521	7,320,264
Broker-dealer and clearing organization receivables	1,222,627	1,780,280
Other assets	72,574	71,040
Financial liabilities:
Deposits	11,649,598	9,032,214
Broker-dealer and clearing organization payables	1,158,628	1,605,518
Short-term borrowings	720,164	1,424,010
Debt	517,170	323,281
Other liabilities	9,603	8,340
Estimate of Fair Value
Financial assets:
Cash and cash equivalents	1,655,877	434,020
Assets segregated for regulatory purposes	194,626	157,436
Securities purchased under agreements to resell	161,457	59,031
Held to maturity securities	359,542	388,930
Loans held for sale	143,256	170,648
Loans held for investment, net	7,930,906	7,567,233
Broker-dealer and clearing organization receivables	1,222,627	1,780,280
Other assets	72,574	71,040
Financial liabilities:
Deposits	11,668,575	9,032,496
Broker-dealer and clearing organization payables	1,158,628	1,605,518
Short-term borrowings	720,164	1,424,010
Debt	517,170	323,281
Other liabilities	9,603	8,340
Estimate of Fair Value | Level 1
Financial assets:
Cash and cash equivalents	1,655,877	434,020
Assets segregated for regulatory purposes	194,626	157,436
Estimate of Fair Value | Level 2
Financial assets:
Securities purchased under agreements to resell	161,457	59,031
Held to maturity securities	359,542	388,930
Loans held for sale	143,256	170,648
Loans held for investment, net	422,891	576,527
Broker-dealer and clearing organization receivables	1,222,627	1,780,280
Other assets	70,461	69,580
Financial liabilities:
Deposits	11,668,575	9,032,496
Broker-dealer and clearing organization payables	1,158,628	1,605,518
Short-term borrowings	720,164	1,424,010
Debt	517,170	323,281
Other liabilities	9,603	8,340
Estimate of Fair Value | Level 3
Financial assets:
Loans held for investment, net	7,508,015	6,990,706
Other assets	$ 2,113	$ 1,460