Summary of Investments (Maturities) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available-For-Sale, Amortized Cost
Due in one year or less	$ 53,911
Due after one year through five years	365,831
Due after five years through 10 years	401,105
Due after 10 years	502,858
Amortized Cost	1,612,626	$ 1,659,760
Available-For-Sale, Fair Value
Due in one year or less	54,386
Due after one year through five years	388,192
Due after five years through 10 years	433,321
Due after 10 years	541,362
Allowance for credit losses	(8)	0
Fair Value	1,717,039	1,719,607
Trading, Amortized Cost
Due in one year or less	1,957
Due after one year through five years	9,221
Due after five years through 10 years	0
Due after 10 years	1,080
Amortized Cost	12,258	11,941
Trading, Fair Value
Due in one year or less	3,935
Due after one year through five years	9,265
Due after five years through 10 years	0
Due after 10 years	1,611
Fair Value	14,811	15,256
Asset-backed securities
Available-For-Sale, Amortized Cost
Securities not categorized by contractual maturity	314
Amortized Cost	314	314
Available-For-Sale, Fair Value
Securities not categorized by contractual maturity	935
Allowance for credit losses	0
Fair Value		750
Mortgage-backed securities
Available-For-Sale, Amortized Cost
Securities not categorized by contractual maturity	14,333
Amortized Cost	14,333	6,250
Available-For-Sale, Fair Value
Securities not categorized by contractual maturity	14,529
Allowance for credit losses	0
Fair Value		$ 6,356
Collateralized mortgage obligations
Available-For-Sale, Amortized Cost
Securities not categorized by contractual maturity	274,274
Available-For-Sale, Fair Value
Securities not categorized by contractual maturity	$ 284,322

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