Investments, Continuous Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2021 USD ($) Secutrity	Dec. 31, 2020 USD ($) Secutrity
Available-for-sale Securities Continuous Unrealized Loss Positions [Abstract]
Number of securities, 12 months or less | Secutrity	174	100
Number of securities, Greater than 12 months | Secutrity	31	24
Number of securities, total | Secutrity	205	124
Available-for-sale Securities Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, 12 months or less	$ 275,685	$ 120,630
Fair value, Greater than 12 months	51,441	33,065
Fair value, total	327,126	153,695
Available-for-sale Securities Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, 12 months or less	(4,448)	(3,405)
Gross unrealized loss, Greater than 12 months	(1,571)	(1,333)
Gross unrealized loss, total	(6,019)	(4,738)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
One year or less, Fair Value	96,999
Excess of one year to five years, Fair Value	368,593
Excess of five years to ten years, Fair Value	165,436
Excess of ten years, Fair Value	16,028
Total contractual maturities, Fair Value	647,056
Asset-backed securities, Fair Value	267,893
Total, Fair Value	914,949	919,692
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
One year or less, Cost or Amortized Cost	96,057
Excess of one year to five years, Cost or Amortized Cost	358,662
Excess of five years to ten years, Cost or Amortized Cost	163,115
Excess of ten years, Cost or Amortized Cost	16,601
Total contractual maturities, Cost or Amortized Cost	634,435
Asset-backed securities, Cost or Amortized Cost	266,574
Cost or amortized cost	$ 901,009	$ 894,468