Other Investments - Components of other investments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Other Investments [Line Items]
Other investments	$ 1,163,863	$ 1,164,564
Hedge funds
Other Investments [Line Items]
Other investments	205,770	331,972
Private investments
Other Investments [Line Items]
Other investments	191,986	175,579
Overseas deposits
Other Investments [Line Items]
Other investments	570,742	493,594
Structured transactions
Other Investments [Line Items]
Other investments	172,982	142,698
Other
Other Investments [Line Items]
Other investments	$ 22,383	$ 20,721