Investments, Continuous Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2020 USD ($) Security	Dec. 31, 2019 USD ($) Security
Available-for-sale Securities Continuous Unrealized Loss Positions [Abstract]
Number of securities, 12 months or less | Security	270	88
Number of securities, Greater than 12 months | Security	20	69
Number of securities, total | Security	290	157
Available-for-sale Securities Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, 12 months or less	$ 225,745	$ 108,387
Fair value, Greater than 12 months	20,410	66,860
Fair value, total	246,155	175,247
Available-for-sale Securities Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, 12 months or less	(25,029)	(2,452)
Gross unrealized loss, Greater than 12 months	(5,017)	(1,100)
Gross unrealized loss, total	(30,046)	(3,552)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
One year or less, Fair Value	97,469
Excess of one year to five years, Fair Value	293,569
Excess of five years to ten years, Fair Value	132,550
Excess of ten years, Fair Value	12,828
Total contractual maturities, Fair Value	536,416
Asset-backed securities, Fair Value	210,440
Total, Fair Value	746,856	795,538
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
One year or less, Cost or Amortized Cost	96,889
Excess of one year to five years, Cost or Amortized Cost	289,041
Excess of five years to ten years, Cost or Amortized Cost	130,161
Excess of ten years, Cost or Amortized Cost	13,198
Total contractual maturities, Cost or Amortized Cost	529,289
Asset-backed securities, Cost or Amortized Cost	228,162
Cost or amortized cost	$ 757,451	$ 783,047