Summary of Investments (Maturities) (Details) - USD ($) $ in Thousands	Jun. 30, 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	192	$ 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	192	199
Available-For-Sale, Amortized Cost
Due in one year or less	141,196
Due after one year through five years	787,370
Due after five years through 10 years	769,568
Due after 10 years	741,552
Amortized Cost	2,974,859	2,956,009
Available-For-Sale, Fair Value
Due in one year or less	142,309
Due after one year through five years	808,996
Due after five years through 10 years	788,131
Due after 10 years	740,576
Fair Value	3,231,385	3,168,542
Trading, Amortized Cost
Due in one year or less	2,958
Due after one year through five years	7,532
Due after five years through 10 years	1,302
Due after 10 years	2,153
Amortized Cost	13,945	13,054
Trading, Fair Value
Due in one year or less	3,300
Due after one year through five years	8,733
Due after five years through 10 years	1,379
Due after 10 years	2,416
Fair Value	15,828	14,390
Fixed Maturities
Held-To-Maturity, Amortized Cost
Amortized Cost	192	198
Held-To-Maturity, Fair Value
Fair Value	192	199
Available-For-Sale, Amortized Cost
Amortized Cost	2,912,326	2,887,505
Available-For-Sale, Fair Value
Fair Value	2,955,079	2,898,126
Trading, Amortized Cost
Amortized Cost	13,945
Trading, Fair Value
Fair Value	15,828
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	4,379
Amortized Cost	4,379	4,407
Available-For-Sale, Fair Value
Securities not categorized by contractual maturity	4,798
Fair Value	4,798	4,270
Mortgage-backed securities
Held-To-Maturity, Amortized Cost
Securities not categorized by contractual maturity	42
Amortized Cost	42	48
Held-To-Maturity, Fair Value
Securities not categorized by contractual maturity	42
Fair Value	42	49
Available-For-Sale, Amortized Cost
Securities not categorized by contractual maturity	15,515
Amortized Cost	15,515	17,288
Available-For-Sale, Fair Value
Securities not categorized by contractual maturity	15,498
Fair Value	15,498	$ 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	452,746
Available-For-Sale, Fair Value
Securities not categorized by contractual maturity	$ 454,771