Investments - Gross Unrealized Losses (Details) $ in Thousands	Dec. 31, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Fair Value
12 Months or Greater	$ 3,962	$ 21,607
Less Than 12 Months	1,008,869	36,510
Total	1,012,831	58,117
Gross Unrealized Losses
12 Months or Greater	(329)	(995)
Less Than 12 Months	(6,372)	(1,054)
Total	$ (6,701)	$ (2,049)
Number of securities in unrealized loss position | Security	563	88
Number of securities in unrealized loss position, twelve months or longer | Security	12	42
U.S. government and agency
Fair Value
12 Months or Greater	$ 0	$ 573
Less Than 12 Months	222,643	0
Total	222,643	573
Gross Unrealized Losses
12 Months or Greater	0	(3)
Less Than 12 Months	(607)	0
Total	(607)	(3)
Other government
Fair Value
12 Months or Greater	1,080	7,351
Less Than 12 Months	37,796	11,000
Total	38,876	18,351
Gross Unrealized Losses
12 Months or Greater	(23)	(345)
Less Than 12 Months	(65)	(500)
Total	(88)	(845)
Corporate
Fair Value
12 Months or Greater	2,754	11,888
Less Than 12 Months	461,772	25,227
Total	464,526	37,115
Gross Unrealized Losses
12 Months or Greater	(306)	(629)
Less Than 12 Months	(4,262)	(553)
Total	(4,568)	(1,182)
Municipal
Fair Value
12 Months or Greater	128	1,783
Less Than 12 Months	12,046	283
Total	12,174	2,066
Gross Unrealized Losses
12 Months or Greater	0	(18)
Less Than 12 Months	(111)	(1)
Total	(111)	(19)
Residential mortgage-backed
Fair Value
12 Months or Greater	0	12
Less Than 12 Months	65,992	0
Total	65,992	12
Gross Unrealized Losses
12 Months or Greater	0	0
Less Than 12 Months	(725)	0
Total	(725)	$ 0
Commercial mortgage-backed
Fair Value
12 Months or Greater	0
Less Than 12 Months	79,606
Total	79,606
Gross Unrealized Losses
12 Months or Greater	0
Less Than 12 Months	(409)
Total	(409)
Asset-backed
Fair Value
12 Months or Greater	0
Less Than 12 Months	129,014
Total	129,014
Gross Unrealized Losses
12 Months or Greater	0
Less Than 12 Months	(193)
Total	$ (193)