Investments, Continuous Unrealized Loss Position (Details) $ in Thousands	Sep. 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	97	275
Number of securities, Greater than 12 months | Security	86	217
Number of securities, total | Security	183	492
Available-for-sale Securities Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, 12 months or less	$ 106,943	$ 282,646
Fair value, Greater than 12 months	58,053	131,001
Fair value, total	164,996	413,647
Available-for-sale Securities Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, 12 months or less	(2,097)	(7,296)
Gross unrealized loss, Greater than 12 months	(831)	(3,497)
Gross unrealized loss, total	(2,928)	(10,793)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
One year or less, Fair Value	67,076
Excess of one year to five years, Fair Value	336,640
Excess of five years to ten years, Fair Value	129,362
Excess of ten years, Fair Value	12,416
Total contractual maturities, Fair Value	545,494
Asset-backed securities, Fair Value	212,347
Total, Fair Value	757,841	592,645
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
One year or less, Cost or Amortized Cost	67,008
Excess of one year to five years, Cost or Amortized Cost	330,035
Excess of five years to ten years, Cost or Amortized Cost	126,528
Excess of ten years, Cost or Amortized Cost	11,645
Total contractual maturities, Cost or Amortized Cost	535,216
Asset-backed securities, Cost or Amortized Cost	209,546
Cost or amortized cost	$ 744,762	$ 600,504