Receivables from and Payables to Brokers, Dealers and Clearing Organizations - Amounts Receivable from Brokers, Dealers and Clearing Organizations (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Brokers and Dealers [Abstract]
Receivable arising from unsettled securities transactions	$ 19,011	$ 52,571
Deposits paid for securities borrowed	74,901	57,572
Receivable from clearing organizations	12,572	4,933
Deposits with clearing organizations	48,329	33,799
Receivable from municipal bond fund	151,053	0
Securities failed to deliver	4,399	1,753
Other	15,143	10,381
Brokers, dealers and clearing organizations	$ 325,408	$ 161,009