Summary of Investments (Maturities) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Held-To-Maturity, Amortized Cost
Due in one year or less	$ 0
Due after one year through five years	150
Due after five years through 10 years	0
Due after 10 years	0
Amortized Cost	150	$ 150
Held-To-Maturity, Fair Value
Due in one year or less	0
Due after one year through five years	150
Due after five years through 10 years	0
Due after 10 years	0
Fair Value	150	150
Available-For-Sale, Amortized Cost
Due in one year or less	48,086
Due after one year through five years	187,837
Due after five years through 10 years	447,086
Due after 10 years	704,798
Amortized Cost	1,577,448	1,516,610
Available-For-Sale, Fair Value
Due in one year or less	48,368
Due after one year through five years	189,107
Due after five years through 10 years	447,598
Due after 10 years	693,603
Fair Value	1,563,996	1,535,070
Trading, Amortized Cost
Due in one year or less	2,359
Due after one year through five years	8,750
Due after five years through 10 years	1,100
Due after 10 years	1,020
Amortized Cost	13,229	14,582
Trading, Fair Value
Due in one year or less	2,629
Due after one year through five years	10,497
Due after five years through 10 years	965
Due after 10 years	1,288
Fair Value	15,379	$ 16,842
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	3,196
Available-For-Sale, Fair Value
Securities not categorized by contractual maturity	3,469
Mortgage-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	8,763
Available-For-Sale, Fair Value
Securities not categorized by contractual maturity	8,682
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	177,682
Available-For-Sale, Fair Value
Securities not categorized by contractual maturity	$ 173,169