Investments, Continuous Unrealized Loss Position (Details) $ in Thousands	Dec. 31, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Number of Securities 12 months or less | Security	88	275
Number of securities, Greater than 12 months | Security	69	217
Number of securities, total | Security	157	492
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, 12 months or less	$ 108,387	$ 282,646
Fair value, Greater than 12 months	66,860	131,001
Fair value, total	175,247	413,647
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, less than 12 months	(2,452)	(7,296)
Gross unrealized loss, 12 months or longer	(1,100)	(3,497)
Gross unrealized loss, total	(3,552)	(10,793)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
One year or less, Fair Value	86,368
Excess of one year to five years, Fair Value	336,689
Excess of five years to ten years, Fair Value	135,169
Excess of ten years, Fair Value	15,079
Total contractual maturities, Fair Value	573,305
Asset-backed securities, Fair Value	222,233
Total, Fair Value	$ 795,538	592,645
Percentage of Available for sale Securities, Debt Maturities, Fair Value [Abstract]
One year or less, Fair Value	10.90%
Excess of one year to five years, Fair Value	42.30%
Excess of five years to ten years, Fair Value	17.00%
Excess of ten years, Fair Value	1.90%
Total contractual maturities, Fair Value	72.10%
Asset-backed securities, Fair Value	27.90%
Total, Fair Value	100.00%
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
One year or less, Cost or Amortized Cost	$ 86,144
Excess of one year to five years, Cost or Amortized Cost	330,026
Excess of five years to ten years, Cost or Amortized Cost	132,985
Excess of ten years, Cost or Amortized Cost	14,337
Total contractual maturities, Cost or Amortized Cost	563,492
Asset-backed securities, Cost or Amortized Cost	219,555
Cost or amortized cost	$ 783,047	$ 600,504
Percentage of Available for sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
One year or less, Cost or Amortized Cost	11.00%
Excess of one year to five years, Cost or Amortized Cost	42.10%
Excess of five years to ten years, Cost or Amortized Cost	17.00%
Excess of ten years, Cost or Amortized Cost	1.90%
Total contractual maturities, Cost or Amortized Cost	72.00%
Asset-backed securities, Cost or Amortized Cost	28.00%
Cost or amortized cost	100.00%