Investments (Other Long Term Investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment Holdings [Line Items]
Other long-term investments	$ 263,074	$ 344,463
Company’s investment in separate accounts
Investment Holdings [Line Items]
Other long-term investments	37,404	34,088
Private equity
Investment Holdings [Line Items]
Other long-term investments	159,643	139,493
Hedge Funds
Investment Holdings [Line Items]
Other long-term investments	53,792	81,104
Real estate-related
Investment Holdings [Line Items]
Other long-term investments	12,226	11,912
Total joint ventures and limited partnerships
Investment Holdings [Line Items]
Other long-term investments	225,661	232,509
Derivatives
Investment Holdings [Line Items]
Other long-term investments	$ 9	$ 77,866