Other Investments - Analysis of net realized and unrealized gains on other investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Investments [Line Items]
Net unrealized gains (losses)	$ 160,727	$ 170,889
Net realized gains (losses)	29,316	20,848	$ 22,111
Hedge funds
Other Investments [Line Items]
Net unrealized gains (losses)	100,103	134,114
Net realized gains (losses)	15,322	12,212	12,769
Private investments
Other Investments [Line Items]
Net unrealized gains (losses)	60,624	36,775
Net realized gains (losses)	$ 13,994	$ 8,636	$ 9,342