Other Investments - Analysis of net realized and unrealized gains on other investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Other Investments [Line Items]
Net unrealized gains (losses)	$ 170,889	$ 227,038
Net realized gains (losses)	20,848	22,111	$ 19,960
Hedge funds
Other Investments [Line Items]
Net unrealized gains (losses)	134,114	188,698
Net realized gains (losses)	12,212	12,769	6,685
Private investment funds
Other Investments [Line Items]
Net unrealized gains (losses)	36,775	38,340
Net realized gains (losses)	$ 8,636	$ 9,342	$ 13,275