Summary of Investments (Maturities) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Available-For-Sale, Amortized Cost
Due in one year or less	$ 67,835
Due after one year through five years	276,483
Due after five years through 10 years	465,814
Due after 10 years	550,192
Cost or Amortized Cost	1,633,875	$ 1,659,760
Fair Value
Due in one year or less	68,253
Due after one year through five years	282,037
Due after five years through 10 years	484,021
Due after 10 years	576,983
Allowance for credit losses	(59)	0
Available-for-sale securities, fair value	1,699,359	1,719,607
Trading, Amortized Cost
Due in one year or less	2,211
Due after one year through five years	7,190
Due after five years through 10 years	0
Due after 10 years	1,101
Amortized Cost	10,502	11,941
Trading, Fair Value
Due in one year or less	3,156
Due after one year through five years	6,694
Due after five years through 10 years	0
Due after 10 years	1,358
Fair Value	11,208	15,256
Asset-backed securities
Available-For-Sale, Amortized Cost
Securities not categorized by contractual maturity	314
Cost or Amortized Cost	314	314
Fair Value
Securities not categorized by contractual maturity	921
Allowance for credit losses	0
Available-for-sale securities, fair value		750
Mortgage-backed securities
Available-For-Sale, Amortized Cost
Securities not categorized by contractual maturity	5,929
Cost or Amortized Cost	5,929	6,250
Fair Value
Securities not categorized by contractual maturity	6,130
Allowance for credit losses	0
Available-for-sale securities, fair value		$ 6,356
Collateralized mortgage obligations
Available-For-Sale, Amortized Cost
Securities not categorized by contractual maturity	267,308
Fair Value
Securities not categorized by contractual maturity	$ 281,073