Investments - Schedule of Gross Unrealized Losses and Fair Values of Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	$ 154,118	$ 438,150
Less Than 12 Months, Unrealized Losses	(1,612)	(7,609)
12 Months or More, Fair Value	111,851	414,704
12 Months or More, Gross Unrealized Losses	(986)	(13,658)
Total, Fair Value	265,969	852,854
Total, Gross Unrealized Losses	(2,598)	(21,267)
State and municipal
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	30,028	19,733
Less Than 12 Months, Unrealized Losses	(741)	(284)
12 Months or More, Fair Value	667	47,018
12 Months or More, Gross Unrealized Losses	(1)	(1,003)
Total, Fair Value	30,695	66,751
Total, Gross Unrealized Losses	(742)	(1,287)
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	23,632	49,180
Less Than 12 Months, Unrealized Losses	(78)	(743)
12 Months or More, Fair Value	37,363	105,778
12 Months or More, Gross Unrealized Losses	(544)	(4,594)
Total, Fair Value	60,995	154,958
Total, Gross Unrealized Losses	(622)	(5,337)
Corporate
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	45,550	243,384
Less Than 12 Months, Unrealized Losses	(365)	(5,089)
12 Months or More, Fair Value	9,933	155,902
12 Months or More, Gross Unrealized Losses	(24)	(5,683)
Total, Fair Value	55,483	399,286
Total, Gross Unrealized Losses	(389)	(10,772)
Commercial mortgage and asset-backed
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	46,434	106,423
Less Than 12 Months, Unrealized Losses	(406)	(1,229)
12 Months or More, Fair Value	56,720	51,805
12 Months or More, Gross Unrealized Losses	(400)	(1,584)
Total, Fair Value	103,154	158,228
Total, Gross Unrealized Losses	(806)	(2,813)
U.S. Treasury securities and obligations guaranteed by the U.S. government
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	8,474	17,618
Less Than 12 Months, Unrealized Losses	(22)	(51)
12 Months or More, Fair Value	7,168	54,201
12 Months or More, Gross Unrealized Losses	(17)	(794)
Total, Fair Value	15,642	71,819
Total, Gross Unrealized Losses	$ (39)	(845)
Redeemable preferred stock
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value		1,812
Less Than 12 Months, Unrealized Losses		(213)
12 Months or More, Fair Value		0
12 Months or More, Gross Unrealized Losses		0
Total, Fair Value		1,812
Total, Gross Unrealized Losses		$ (213)