Investments (Maturities of Fixed Maturity Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities [Abstract]
Due in One Year or Less	$ 16,943
Due After One Year Through Five Years	390,698
Due After Five Years Through Ten Years	508,270
Due After Ten Years	506,309
Structured Securities	423,139
Amortized cost	1,845,359	$ 1,870,654
Due in One Year or Less	17,206
Due After One Year Through Five Years	401,578
Due After Five Years Through Ten Years	508,264
Due After Ten Years	511,426
Structured Securities	425,294
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $1,845,359 and $1,870,654, respectively)	$ 1,863,768	$ 1,878,514