Investments (Fair Value Below Amortized Cost of Gross Unrealized Losses for Fixed Maturity and Equity Securities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Gross unrealized losses with number of securities [Line Items]
Available-for-sale Securities, Gross Unrealized Losses	$ 116,993	$ 374,857
Percentage Of Total Gross Unrealized Losses	100.00%	100.00%
Less Than 20 Percent [Member]
Gross unrealized losses with number of securities [Line Items]
Available-for-sale Securities, Gross Unrealized Losses	$ 113,466	$ 337,831
Percentage Of Total Gross Unrealized Losses	97.00%	90.10%
More Than Or Equal To 20 Percent For Less Than Six Months [Member]
Gross unrealized losses with number of securities [Line Items]
Available-for-sale Securities, Gross Unrealized Losses	$ 689	$ 19,438
Percentage Of Total Gross Unrealized Losses	0.60%	5.20%
More Than Or Equal To 20 Percent For Six Months Or Greater [Member]
Gross unrealized losses with number of securities [Line Items]
Available-for-sale Securities, Gross Unrealized Losses	$ 2,838	$ 17,588
Percentage Of Total Gross Unrealized Losses	2.40%	4.70%