Financial Assets and Liabilities - Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Cash and cash equivalents	$ 736,047	$ 532,887	$ 181,415	$ 163,235
Securities borrowed	1,399,684	1,471,172
Securities purchased under agreements to resell	15,475	0
Receivables from broker dealers and clearing organizations	1,101,449	972,018
Total assets	7,380,978	7,320,006
Liabilities
Short-term borrowings	15,128	27,883
Long-term borrowings	907,037	1,388,548
Securities loaned	1,130,039	754,687
Securities sold under agreements to repurchase	281,861	390,642
Payables to broker dealers and clearing organizations	567,441	716,205
Total liabilities	5,886,279	6,168,428
Carrying Value
Assets
Cash and cash equivalents	736,047	532,887
Securities borrowed	1,399,684	1,471,172
Securities purchased under agreements to resell	15,475
Receivables from broker dealers and clearing organizations	1,101,449	972,018
Total assets	3,252,655	2,976,077
Liabilities
Short-term borrowings	15,128	27,883
Long-term borrowings	907,037	1,388,548
Securities loaned	1,130,039	754,687
Securities sold under agreements to repurchase	281,861	390,642
Payables to broker dealers and clearing organizations	567,441	716,205
Total liabilities	2,901,506	3,277,965
Fair Value
Assets
Cash and cash equivalents	736,047	532,887
Securities borrowed	1,399,684	1,471,172
Securities purchased under agreements to resell	15,475
Receivables from broker dealers and clearing organizations	1,101,449	972,018
Total assets	3,252,655	2,976,077
Liabilities
Short-term borrowings	15,128	27,883
Long-term borrowings	916,465	1,465,489
Securities loaned	1,130,039	754,687
Securities sold under agreements to repurchase	281,861	390,642
Payables to broker dealers and clearing organizations	567,441	716,205
Total liabilities	2,910,934	3,354,906
Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Cash and cash equivalents	736,047	532,887
Securities borrowed	0	0
Securities purchased under agreements to resell	0
Receivables from broker dealers and clearing organizations	71,288	36,513
Total assets	807,335	569,400
Liabilities
Short-term borrowings	0	0
Long-term borrowings	0	0
Securities loaned	0	0
Securities sold under agreements to repurchase	0	0
Payables to broker dealers and clearing organizations	1,031	2,925
Total liabilities	1,031	2,925
Fair Value | Significant Other Observable Inputs (Level 2)
Assets
Cash and cash equivalents	0	0
Securities borrowed	1,399,684	1,471,172
Securities purchased under agreements to resell	15,475
Receivables from broker dealers and clearing organizations	1,030,161	935,505
Total assets	2,445,320	2,406,677
Liabilities
Short-term borrowings	15,128	27,883
Long-term borrowings	916,465	1,465,489
Securities loaned	1,130,039	754,687
Securities sold under agreements to repurchase	281,861	390,642
Payables to broker dealers and clearing organizations	566,410	713,280
Total liabilities	2,909,903	3,351,981
Fair Value | Significant Unobservable Inputs (Level 3)
Assets
Cash and cash equivalents	0	0
Securities borrowed	0	0
Securities purchased under agreements to resell	0
Receivables from broker dealers and clearing organizations	0	0
Total assets	0	0
Liabilities
Short-term borrowings	0	0
Long-term borrowings	0	0
Securities loaned	0	0
Securities sold under agreements to repurchase	0	0
Payables to broker dealers and clearing organizations	0	0
Total liabilities	$ 0	$ 0