Investments, Continuous Unrealized Loss Position (Details) $ in Thousands		Dec. 31, 2018 USD ($) Security		Dec. 31, 2017 USD ($) Security
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Number of securities, total | Security		492		636
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, total		$ 413,647		$ 435,713
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, total		(10,793)		$ (7,056)
Fixed Income Securities [Member]
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
One year or less, Fair Value		45,858
Excess of one year to five years, Fair Value		287,506
Excess of five years to ten years, Fair Value		101,605
Excess of ten years, Fair Value		6,641
Total contractual maturities, Fair Value		441,610
Asset-backed securities, Fair Value		151,035
Total, Fair Value		$ 592,645
Percentage of Available for sale Securities, Debt Maturities, Fair Value [Abstract]
One year or less, Fair Value		7.70%
Excess of one year to five years, Fair Value		48.50%
Excess of five years to ten years, Fair Value		17.10%
Excess of ten years, Fair Value		1.20%
Total contractual maturities, Fair Value		74.50%
Asset-backed securities, Fair Value		25.50%
Total, Fair Value		100.00%
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
One year or less, Cost or Amortized Cost		$ 46,150
Excess of one year to five years, Cost or Amortized Cost		290,743
Excess of five years to ten years, Cost or Amortized Cost		104,571
Excess of ten years, Cost or Amortized Cost		6,410
Total contractual maturities, Cost or Amortized Cost		447,874
Asset-backed securities, Cost or Amortized Cost		152,630
Cost or amortized cost		$ 600,504
Percentage of Available for sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
One year or less, Cost or Amortized Cost		7.70%
Excess of one year to five years, Cost or Amortized Cost		48.40%
Excess of five years to ten years, Cost or Amortized Cost		17.40%
Excess of ten years, Cost or Amortized Cost		1.10%
Total contractual maturities, Cost or Amortized Cost		74.60%
Asset-backed securities, Cost or Amortized Cost		25.40%
Cost or amortized cost		100.00%
Total Fixed Income Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Number of Securities 12 months or less | Security		275		459
Number of securities, Greater than 12 months | Security		217		112
Number of securities, total | Security		492		571
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, 12 months or less		$ 282,646		$ 313,421
Fair value, Greater than 12 months		131,001		75,638
Fair value, total		413,647		389,059
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, less than 12 months		(7,296)		(2,683)
Gross unrealized loss, 12 months or longer		(3,497)		(2,155)
Gross unrealized loss, total		(10,793)		(4,838)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Total, Fair Value		592,645	[1]	521,853
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Cost or amortized cost		$ 600,504	[1]	$ 521,017
Total Equity Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Number of Securities 12 months or less | Security	[1]	0		65
Number of securities, Greater than 12 months | Security	[1]	0		0
Number of securities, total | Security	[1]	0		65
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, 12 months or less	[1]	$ 0		$ 46,654
Fair value, Greater than 12 months	[1]	0		0
Fair value, total	[1]	0		46,654
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, less than 12 months	[1]	0		(2,218)
Gross unrealized loss, 12 months or longer	[1]	0		0
Gross unrealized loss, total	[1]	$ 0		$ (2,218)

[1] Effective January 1, 2018, the Company adopted ASU 2016-01 and equity securities are no longer classified as available-for-sale. Prior periods have not been restated to conform to the current presentation. See Note A - Summary of Significant Accounting Policies - Recently Adopted Accounting Pronouncements for further discussion.