Summary of Investments (Maturities) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Held-To-Maturity, Amortized Cost
Due in one year or less	$ 150
Due after one year through five years	0
Due after five years through 10 years	0
Due after 10 years	0
Amortized Cost	195	$ 198
Held-To-Maturity, Fair Value
Due in one year or less	150
Due after one year through five years	0
Due after five years through 10 years	0
Due after 10 years	0
Fair Value	196	199
Available-For-Sale, Amortized Cost
Due in one year or less	125,861
Due after one year through five years	814,770
Due after five years through 10 years	802,379
Due after 10 years	716,755
Amortized Cost	2,982,501	2,956,009
Available-For-Sale, Fair Value
Due in one year or less	126,941
Due after one year through five years	835,999
Due after five years through 10 years	812,933
Due after 10 years	703,397
Fair Value	3,207,872	3,168,542
Trading, Amortized Cost
Due in one year or less	2,593
Due after one year through five years	7,213
Due after five years through 10 years	1,302
Due after 10 years	2,156
Amortized Cost	13,264	13,054
Trading, Fair Value
Due in one year or less	2,815
Due after one year through five years	8,396
Due after five years through 10 years	1,377
Due after 10 years	2,383
Fair Value	14,971	14,390
Fixed Maturities
Held-To-Maturity, Amortized Cost
Amortized Cost	195	198
Held-To-Maturity, Fair Value
Fair Value	196	199
Available-For-Sale, Amortized Cost
Amortized Cost	2,914,524	2,887,505
Available-For-Sale, Fair Value
Fair Value	2,932,188	2,898,126
Trading, Amortized Cost
Amortized Cost	13,264
Trading, Fair Value
Fair Value	14,971
Asset-backed securities
Held-To-Maturity, Amortized Cost
Securities not categorized by contractual maturity	0
Held-To-Maturity, Fair Value
Securities not categorized by contractual maturity	0
Available-For-Sale, Amortized Cost
Securities not categorized by contractual maturity	4,367
Amortized Cost	4,367	4,407
Available-For-Sale, Fair Value
Securities not categorized by contractual maturity	4,328
Fair Value	4,328	4,270
Mortgage-backed securities
Held-To-Maturity, Amortized Cost
Securities not categorized by contractual maturity	45
Amortized Cost	45	48
Held-To-Maturity, Fair Value
Securities not categorized by contractual maturity	46
Fair Value	46	49
Available-For-Sale, Amortized Cost
Securities not categorized by contractual maturity	16,366
Amortized Cost	16,366	17,288
Available-For-Sale, Fair Value
Securities not categorized by contractual maturity	16,282
Fair Value	16,282	$ 17,248
Collateralized mortgage obligations
Held-To-Maturity, Amortized Cost
Securities not categorized by contractual maturity	0
Held-To-Maturity, Fair Value
Securities not categorized by contractual maturity	0
Available-For-Sale, Amortized Cost
Securities not categorized by contractual maturity	434,026
Available-For-Sale, Fair Value
Securities not categorized by contractual maturity	$ 432,308