Summary of Investments (Maturities) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Held-To-Maturity, Amortized Cost
Due in one year or less	$ 150
Due after one year through five years	450
Due after five years through 10 years	0
Due after 10 years	0
Amortized Cost	652	$ 672
Held-To-Maturity, Fair Value
Due in one year or less	150
Due after one year through five years	452
Due after five years through 10 years	0
Due after 10 years	0
Fair Value	655	675
Available-For-Sale, Amortized Cost
Due in one year or less	99,301
Due after one year through five years	820,503
Due after five years through 10 years	879,226
Due after 10 years	588,600
Amortized Cost	2,868,983	2,861,583
Available-For-Sale, Fair Value
Due in one year or less	100,197
Due after one year through five years	851,672
Due after five years through 10 years	926,253
Due after 10 years	609,336
Fair Value	3,167,565	3,061,208
Trading, Amortized Cost
Due in one year or less	2,430
Due after one year through five years	6,036
Due after five years through 10 years	961
Due after 10 years	2,160
Amortized Cost	11,587	11,475
Trading, Fair Value
Due in one year or less	2,919
Due after one year through five years	6,905
Due after five years through 10 years	1,133
Due after 10 years	2,296
Fair Value	13,253	12,622
Fixed Maturities
Held-To-Maturity, Amortized Cost
Amortized Cost	652	672
Held-To-Maturity, Fair Value
Fair Value	655	675
Available-For-Sale, Amortized Cost
Amortized Cost	2,801,074	2,793,069
Available-For-Sale, Fair Value
Fair Value	2,916,464	2,824,961
Trading, Amortized Cost
Amortized Cost	11,587
Trading, Fair Value
Fair Value	13,253
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	4,491
Amortized Cost	4,491	5,461
Available-For-Sale, Fair Value
Securities not categorized by contractual maturity	4,578
Fair Value	4,578	5,666
Mortgage-backed securities
Held-To-Maturity, Amortized Cost
Securities not categorized by contractual maturity	52
Amortized Cost	52	72
Held-To-Maturity, Fair Value
Securities not categorized by contractual maturity	53
Fair Value	53	74
Available-For-Sale, Amortized Cost
Securities not categorized by contractual maturity	18,422
Amortized Cost	18,422	16,413
Available-For-Sale, Fair Value
Securities not categorized by contractual maturity	18,879
Fair Value	18,879	$ 16,738
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	390,531
Available-For-Sale, Fair Value
Securities not categorized by contractual maturity	$ 405,549