Assets Segregated or Held in Separate Accounts Under Federal or Other Regulations - (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Regulated Operations [Abstract]
Cash and cash equivalents segregated under federal or other regulations	$ 3,361		$ 183,082
Receivables from brokers, dealers and clearing organizations	0	[1]	304,294	[1]
Total assets segregated or held in separate accounts under federal or other regulations	$ 3,361		$ 487,376

[1] Segregated assets included within Receivable from brokers, dealers and clearing organizations comprise cash and cash equivalents and U.S. government obligations primarily held as deposits with exchange clearing organizations.