Investments, Fixed Income Investments by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
One year or less, Fair Value	$ 117,017
Excess of one year to five years, Fair Value	359,314
Excess of five years to ten years, Fair Value	161,700
Excess of ten years, Fair Value	21,871
Total contractual maturities, Fair Value	659,902
Asset-backed securities, Fair Value	259,790
Total, Fair Value	$ 919,692	$ 795,538
Percentage of Available for sale Securities, Debt Maturities, Fair Value [Abstract]
One year or less, Fair Value	12.70%
Excess of one year to five years, Fair Value	39.10%
Excess of five years to ten years, Fair Value	17.60%
Excess of ten years, Fair Value	2.40%
Total contractual maturities, Fair Value	71.80%
Asset-backed securities, Fair Value	28.20%
Total, Fair Value	100.00%
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
One year or less, Cost or Amortized Cost	$ 115,774
Excess of one year to five years, Cost or Amortized Cost	346,150
Excess of five years to ten years, Cost or Amortized Cost	151,446
Excess of ten years, Cost or Amortized Cost	22,264
Total contractual maturities, Cost or Amortized Cost	635,634
Asset-backed securities, Cost or Amortized Cost	258,834
Cost or amortized cost	$ 894,468	$ 783,047
Percentage of Available for sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
One year or less, Cost or Amortized Cost	12.90%
Excess of one year to five years, Cost or Amortized Cost	38.70%
Excess of five years to ten years, Cost or Amortized Cost	16.90%
Excess of ten years, Cost or Amortized Cost	2.60%
Total contractual maturities, Cost or Amortized Cost	71.10%
Asset-backed securities, Cost or Amortized Cost	28.90%
Cost or amortized cost	100.00%