Investments - Schedule of Credit Quality of Investment Securities With Contractual Maturities or The Issuer of Such Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 867,249	$ 855,284
Percentage of Total Fair Value	100.00%	100.00%
AAA [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 388,197	$ 372,442
Percentage of Total Fair Value	44.80%	43.60%
AA [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 95,823	$ 99,103
Percentage of Total Fair Value	11.00%	11.60%
A [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 242,406	$ 238,766
Percentage of Total Fair Value	28.00%	27.90%
BBB [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 136,794	$ 143,889
Percentage of Total Fair Value	15.80%	16.80%
BB and Below [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 242	$ 0
Percentage of Total Fair Value	0.00%	0.00%
Not Rated Available [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 3,787	$ 1,084
Percentage of Total Fair Value	0.40%	0.10%