Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Financial Assets
Beginning balance	$ 371,200,000	$ 340,200,000	$ 371,400,000	$ 355,100,000
Transfers into Level 3	73,500,000	31,600,000	145,700,000	58,800,000
Transfers out of Level 3	0	(42,700,000)	(8,000,000.0)	(75,900,000)
Total gains or losses
Net investment losses included in net income related to financial assets	(1,300,000)	0	(2,300,000)	0
Net unrealized investment gains (losses) included in OCI	(10,900,000)	4,800,000	(24,900,000)	1,600,000
Purchases	4,900,000	0	4,900,000	0
Issuances	0	0	0	0
Sales	(2,100,000)	0	(2,100,000)	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(15,300,000)	(9,000,000.0)	(64,700,000)	(14,700,000)
Ending balance	420,000,000.0	324,900,000	420,000,000.0	324,900,000
Financial Liabilities
Beginning balance	99,100,000	107,600,000	106,600,000	104,500,000
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Total gains or losses
Net investment (gains) losses included in net income related to financial liabilities	(4,600,000)	(3,200,000)	(9,800,000)	(7,500,000)
Net unrealized investment gains (losses) included in OCI	0	0	0	0
Purchases	0	0	0	0
Issuances	1,200,000	1,200,000	2,100,000	1,900,000
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(2,500,000)	(3,100,000)	(5,700,000)	(5,000,000.0)
Ending balance	93,200,000	108,900,000	93,200,000	108,900,000
Municipal bonds
Financial Assets
Beginning balance	54,100,000	58,600,000	60,800,000	59,600,000
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	(3,200,000)	0
Total gains or losses
Net investment losses included in net income related to financial assets	0	0	0	0
Net unrealized investment gains (losses) included in OCI	(2,800,000)	300,000	(6,200,000)	(600,000)
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(300,000)	(300,000)	(400,000)	(400,000)
Ending balance	51,000,000.0	58,600,000	51,000,000.0	58,600,000
Corporate bonds
Financial Assets
Beginning balance	226,000,000.0	149,100,000	210,300,000	155,800,000
Transfers into Level 3	56,000,000.0	28,500,000	123,400,000	52,600,000
Transfers out of Level 3	0	(29,400,000)	0	(56,700,000)
Total gains or losses
Net investment losses included in net income related to financial assets	0	0	0	0
Net unrealized investment gains (losses) included in OCI	(4,900,000)	3,200,000	(11,300,000)	1,200,000
Purchases	4,900,000	0	4,900,000	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(11,500,000)	(900,000)	(56,800,000)	(2,400,000)
Ending balance	270,500,000	150,500,000	270,500,000	150,500,000
Mortgage and Asset Backed Securities
Financial Assets
Beginning balance	89,800,000	132,200,000	98,900,000	139,400,000
Transfers into Level 3	17,500,000	3,100,000	22,300,000	6,200,000
Transfers out of Level 3	0	(13,300,000)	(4,800,000)	(19,200,000)
Total gains or losses
Net investment losses included in net income related to financial assets	(1,400,000)	0	(2,300,000)	0
Net unrealized investment gains (losses) included in OCI	(3,200,000)	1,300,000	(7,400,000)	1,000,000.0
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	(2,100,000)	0	(2,100,000)	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(3,500,000)	(7,800,000)	(7,500,000)	(11,900,000)
Ending balance	97,100,000	115,500,000	97,100,000	115,500,000
Total Fixed Maturity Securities
Financial Assets
Beginning balance	369,900,000	339,900,000	370,000,000.0	354,800,000
Transfers into Level 3	73,500,000	31,600,000	145,700,000	58,800,000
Transfers out of Level 3	0	(42,700,000)	(8,000,000.0)	(75,900,000)
Total gains or losses
Net investment losses included in net income related to financial assets	(1,400,000)	0	(2,300,000)	0
Net unrealized investment gains (losses) included in OCI	(10,900,000)	4,800,000	(24,900,000)	1,600,000
Purchases	4,900,000	0	4,900,000	0
Issuances	0	0	0	0
Sales	(2,100,000)	0	(2,100,000)	0
Settlements	0	0	0	0
Paydowns, maturities and distributions	(15,300,000)	(9,000,000.0)	(64,700,000)	(14,700,000)
Ending balance	418,600,000	324,600,000	418,600,000	324,600,000
Equity securities
Financial Assets
Beginning balance	1,300,000	300,000	1,400,000	300,000
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Total gains or losses
Net investment losses included in net income related to financial assets	100,000	0	0	0
Net unrealized investment 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	$ 1,400,000	$ 300,000	$ 1,400,000	$ 300,000