Investments (Fair Value Below Amortized Cost of Gross Unrealized Losses for Fixed Maturity and Equity Securities) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Gross unrealized losses with number of securities [Line Items]
Gross Unrealized Losses	$ 592,208	$ 116,993
% of Total	100.00%	100.00%
Less than 20%
Gross unrealized losses with number of securities [Line Items]
Gross Unrealized Losses	$ 571,151	$ 113,466
% of Total	96.40%	97.00%
20% or more for less than six months
Gross unrealized losses with number of securities [Line Items]
Gross Unrealized Losses	$ 21,045	$ 689
% of Total	3.60%	0.60%
20% or more for six months or greater
Gross unrealized losses with number of securities [Line Items]
Gross Unrealized Losses	$ 12	$ 2,838
% of Total	0.00%	2.40%