Investments, Continuous Unrealized Loss Position (Details) $ in Thousands	Jun. 30, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Available-for-sale Securities Continuous Unrealized Loss Positions [Abstract]
Number of securities, 12 months or less | Security	81	275
Number of securities, Greater than 12 months | Security	149	217
Number of securities, total | Security	230	492
Available-for-sale Securities Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, 12 months or less	$ 76,363	$ 282,646
Fair value, Greater than 12 months	95,614	131,001
Fair value, total	171,977	413,647
Available-for-sale Securities Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, 12 months or less	(1,166)	(7,296)
Gross unrealized loss, Greater than 12 months	(914)	(3,497)
Gross unrealized loss, total	(2,080)	(10,793)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
One year or less, Fair Value	99,531
Excess of one year to five years, Fair Value	342,358
Excess of five years to ten years, Fair Value	89,596
Excess of ten years, Fair Value	11,517
Total contractual maturities, Fair Value	543,002
Asset-backed securities, Fair Value	194,146
Total, Fair Value	737,148	592,645
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
One year or less, Cost or Amortized Cost	99,574
Excess of one year to five years, Cost or Amortized Cost	336,988
Excess of five years to ten years, Cost or Amortized Cost	87,051
Excess of ten years, Cost or Amortized Cost	10,894
Total contractual maturities, Cost or Amortized Cost	534,507
Asset-backed securities, Cost or Amortized Cost	191,546
Cost or amortized cost	$ 726,053	$ 600,504