Summary of Investments (Maturities) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Available-For-Sale, Amortized Cost
Due in one year or less	$ 59,024
Due after one year through five years	253,553
Due after five years through 10 years	490,802
Due after 10 years	578,970
Amortized Cost	1,663,984	$ 1,761,289
Fair Value
Due in one year or less	59,242
Due after one year through five years	260,509
Due after five years through 10 years	513,962
Due after 10 years	605,618
Available-for-sale securities, fair value	1,729,286	1,749,488
Trading, Amortized Cost
Due in one year or less	4,235
Due after one year through five years	5,859
Due after five years through 10 years	0
Due after 10 years	2,533
Amortized Cost	12,627	11,277
Trading, Fair Value
Due in one year or less	5,797
Due after one year through five years	8,120
Due after five years through 10 years	0
Due after 10 years	2,963
Fair Value	16,880	13,240
Asset-backed securities
Available-For-Sale, Amortized Cost
Securities not categorized by contractual maturity	3,240
Amortized Cost	3,240	3,256
Fair Value
Securities not categorized by contractual maturity	3,660
Available-for-sale securities, fair value	3,660	3,495
Mortgage-backed securities
Available-For-Sale, Amortized Cost
Securities not categorized by contractual maturity	6,649
Amortized Cost	6,649	7,642
Fair Value
Securities not categorized by contractual maturity	6,752
Available-for-sale securities, fair value	6,752	$ 7,424
Collateralized mortgage obligations
Available-For-Sale, Amortized Cost
Securities not categorized by contractual maturity	271,746
Fair Value
Securities not categorized by contractual maturity	$ 279,543

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