Other Investments - Components of other investments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Other Investments [Line Items]
Other investments	$ 1,248,439	$ 1,164,630
Alternative funds
Other Investments [Line Items]
Other investments	640,847	645,187
Arbitrage
Other Investments [Line Items]
Other investments	179,821	158,220
Directional
Other Investments [Line Items]
Other investments	336,082	357,469
Event driven
Other Investments [Line Items]
Other investments	56,238	56,873
Multi-style
Other Investments [Line Items]
Other investments	68,706	72,625
Private investment funds
Other Investments [Line Items]
Other investments	129,243	94,064
Overseas deposits
Other Investments [Line Items]
Other investments	92,223	111,302
Structured transactions
Other Investments [Line Items]
Other investments	350,563	294,048
Other
Other Investments [Line Items]
Other investments	$ 35,563	$ 20,029