Investments, Continuous Unrealized Loss Position (Details) $ in Thousands	Mar. 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	85	275
Number of securities, Greater than 12 months | Security	253	217
Number of securities, total | Security	338	492
Available-for-sale Securities Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, 12 months or less	$ 96,462	$ 282,646
Fair value, Greater than 12 months	208,253	131,001
Fair value, total	304,715	413,647
Available-for-sale Securities Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, 12 months or less	(1,570)	(7,296)
Gross unrealized loss, Greater than 12 months	(2,062)	(3,497)
Gross unrealized loss, total	(3,632)	(10,793)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
One year or less, Fair Value	95,201
Excess of one year to five years, Fair Value	315,983
Excess of five years to ten years, Fair Value	81,812
Excess of ten years, Fair Value	6,630
Total contractual maturities, Fair Value	499,626
Asset-backed securities, Fair Value	172,347
Total, Fair Value	671,973	592,645
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
One year or less, Cost or Amortized Cost	95,347
Excess of one year to five years, Cost or Amortized Cost	314,909
Excess of five years to ten years, Cost or Amortized Cost	80,997
Excess of ten years, Cost or Amortized Cost	6,233
Total contractual maturities, Cost or Amortized Cost	497,486
Asset-backed securities, Cost or Amortized Cost	171,642
Cost or amortized cost	$ 669,128	$ 600,504