Investments (Fair Value Below Amortized Cost of Gross Unrealized Losses for Fixed Maturity and Equity Securities) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Gross unrealized losses with number of securities [Line Items]
Gross Unrealized Losses	$ 163,210	$ 374,857
% of Total	100.00%	100.00%
Less than 20%
Gross unrealized losses with number of securities [Line Items]
Gross Unrealized Losses	$ 140,313	$ 337,831
% of Total	86.00%	90.10%
20% or more for less than six months
Gross unrealized losses with number of securities [Line Items]
Gross Unrealized Losses	$ 3,407	$ 19,438
% of Total	2.10%	5.20%
20% or more for six months or greater
Gross unrealized losses with number of securities [Line Items]
Gross Unrealized Losses	$ 19,490	$ 17,588
% of Total	11.90%	4.70%