Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Financial Assets
Beginning balance	$ 446,000,000.0	$ 371,200,000	$ 425,300,000	$ 371,400,000
Transfers into Level 3	30,600,000	73,500,000	36,900,000	145,700,000
Transfers out of Level 3	0	0	0	(8,000,000.0)
Total gains or losses
Net gains/losses included in net income	0	(1,300,000)	0	(2,300,000)
Net unrealized gains/losses included in OCI	(3,700,000)	(10,900,000)	(1,800,000)	(24,900,000)
Purchases	16,100,000	4,900,000	35,400,000	4,900,000
Issuances	0	0	0	0
Sales	(5,100,000)	(2,100,000)	(7,700,000)	(2,100,000)
Settlements	0	0	0	0
Paydowns, maturities and distributions	(5,100,000)	(15,300,000)	(9,300,000)	(64,700,000)
Ending balance	478,800,000	420,000,000.0	478,800,000	420,000,000.0
Financial Liabilities
Beginning balance	91,200,000	103,100,000	91,300,000	111,400,000
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Total gains or losses
Net gains/losses included in net income	(3,400,000)	(2,000,000.0)	(2,500,000)	(7,600,000)
Net unrealized gains/losses included in OCI	(900,000)	(2,000,000.0)	(600,000)	(2,400,000)
Purchases	0	0	0	0
Issuances	2,200,000	1,200,000	4,300,000	2,100,000
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(5,500,000)	(2,500,000)	(8,900,000)	(5,700,000)
Ending balance	83,600,000	97,800,000	83,600,000	97,800,000
Municipal bonds
Financial Assets
Beginning balance	55,400,000	54,100,000	54,400,000	60,800,000
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	(3,200,000)
Total gains or losses
Net gains/losses included in net income	0	0	0	0
Net unrealized gains/losses included in OCI	(800,000)	(2,800,000)	500,000	(6,200,000)
Purchases	9,500,000	0	9,500,000	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(200,000)	(300,000)	(500,000)	(400,000)
Ending balance	63,900,000	51,000,000.0	63,900,000	51,000,000.0
Corporate bonds
Financial Assets
Beginning balance	284,000,000.0	226,000,000.0	261,300,000	210,300,000
Transfers into Level 3	30,200,000	56,000,000.0	36,100,000	123,400,000
Transfers out of Level 3	0	0	0	0
Total gains or losses
Net gains/losses included in net income	0	0	0	0
Net unrealized gains/losses included in OCI	(2,200,000)	(4,900,000)	(1,200,000)	(11,300,000)
Purchases	6,600,000	4,900,000	25,600,000	4,900,000
Issuances	0	0	0	0
Sales	(5,100,000)	0	(7,700,000)	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(2,000,000.0)	(11,500,000)	(2,600,000)	(56,800,000)
Ending balance	311,500,000	270,500,000	311,500,000	270,500,000
Mortgage Backed and Other Asset- Backed Securities
Financial Assets
Beginning balance	104,600,000	89,800,000	107,600,000	98,900,000
Transfers into Level 3	400,000	17,500,000	800,000	22,300,000
Transfers out of Level 3	0	0	0	(4,800,000)
Total gains or losses
Net gains/losses included in net income	0	(1,400,000)	0	(2,300,000)
Net unrealized gains/losses included in OCI	(700,000)	(3,200,000)	(1,100,000)	(7,400,000)
Purchases	0	0	300,000	0
Issuances	0	0	0	0
Sales	0	(2,100,000)	0	(2,100,000)
Settlements	0	0	0	0
Paydowns, maturities and distributions	(2,900,000)	(3,500,000)	(6,200,000)	(7,500,000)
Ending balance	101,400,000	97,100,000	101,400,000	97,100,000
Total Fixed Maturity Securities
Financial Assets
Beginning balance	444,000,000.0	369,900,000	423,300,000	370,000,000.0
Transfers into Level 3	30,600,000	73,500,000	36,900,000	145,700,000
Transfers out of Level 3	0	0	0	(8,000,000.0)
Total gains or losses
Net gains/losses included in net income	0	(1,400,000)	0	(2,300,000)
Net unrealized gains/losses included in OCI	(3,700,000)	(10,900,000)	(1,800,000)	(24,900,000)
Purchases	16,100,000	4,900,000	35,400,000	4,900,000
Issuances	0	0	0	0
Sales	(5,100,000)	(2,100,000)	(7,700,000)	(2,100,000)
Settlements	0	0	0	0
Paydowns, maturities and distributions	(5,100,000)	(15,300,000)	(9,300,000)	(64,700,000)
Ending balance	476,800,000	418,600,000	476,800,000	418,600,000
Equity securities
Financial Assets
Beginning balance	2,000,000.0	1,300,000	2,000,000.0	1,400,000
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Total gains or losses
Net gains/losses included in net income	0	100,000	0	0
Net unrealized gains/losses included in OCI	0	0	0	0
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	0	0	0	0
Ending balance	$ 2,000,000.0	$ 1,400,000	$ 2,000,000.0	$ 1,400,000