Investment Operations Change in unrealized appreciation/depreciation of investments in AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Change in unrealized appreciation/depreciation of investments	$ 510,591	$ (357,950)	$ 192,580
Debt Securities
Debt Securities, Available-for-sale [Line Items]
Change in unrealized appreciation/depreciation of investments	510,591	(357,950)	187,639
Equity Securities
Debt Securities, Available-for-sale [Line Items]
Change in unrealized appreciation/depreciation of investments	$ 0	$ 0	$ 4,941