Investment Operations Change in unrealized appreciation/depreciation of investments in AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Change in unrealized appreciation/depreciation of investments	$ 86,380	$ (331,290)	$ 339,780
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Change in unrealized appreciation/depreciation of investments	89,222	(331,408)	336,051
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Change in unrealized appreciation/depreciation of investments	$ (2,842)	$ 118	$ 3,729