Investments - Amortized Cost, Gross Unrealized Gains and Losses, Fair Value and OTTI (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities available for sale, amortized cost	$ 6,863.6	$ 6,903.9
Fixed maturity securities available for sale, allowances for credit losses	0.0	0.0
Fixed maturity securities, gross unrealized gains	17.1	346.6
Fixed maturity securities, gross unrealized losses	(742.8)	(35.2)
Fixed maturity securities, fair value	6,137.9	7,215.3
U.S. government and government agencies and authorities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities available for sale, amortized cost	102.0	83.0
Fixed maturity securities available for sale, allowances for credit losses	0.0	0.0
Fixed maturity securities, gross unrealized gains	0.3	2.1
Fixed maturity securities, gross unrealized losses	(7.1)	(0.1)
Fixed maturity securities, fair value	95.2	85.0
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities available for sale, amortized cost	160.8	142.2
Fixed maturity securities available for sale, allowances for credit losses	0.0	0.0
Fixed maturity securities, gross unrealized gains	0.9	7.0
Fixed maturity securities, gross unrealized losses	(16.6)	(0.7)
Fixed maturity securities, fair value	145.1	148.5
Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities available for sale, amortized cost	403.1	436.0
Fixed maturity securities available for sale, allowances for credit losses	0.0	0.0
Fixed maturity securities, gross unrealized gains	0.6	5.9
Fixed maturity securities, gross unrealized losses	(21.0)	(4.2)
Fixed maturity securities, fair value	382.7	437.7
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities available for sale, amortized cost	670.1	411.1
Fixed maturity securities available for sale, allowances for credit losses	0.0	0.0
Fixed maturity securities, gross unrealized gains	3.0	14.2
Fixed maturity securities, gross unrealized losses	(46.9)	(2.3)
Fixed maturity securities, fair value	626.2	423.0
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities available for sale, amortized cost	461.7	466.7
Fixed maturity securities available for sale, allowances for credit losses	0.0	0.0
Fixed maturity securities, gross unrealized gains	0.0	10.3
Fixed maturity securities, gross unrealized losses	(50.5)	(3.3)
Fixed maturity securities, fair value	411.2	473.7
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities available for sale, amortized cost	515.5	578.4
Fixed maturity securities available for sale, allowances for credit losses	0.0	0.0
Fixed maturity securities, gross unrealized gains	0.5	25.2
Fixed maturity securities, gross unrealized losses	(57.7)	(1.7)
Fixed maturity securities, fair value	458.3	601.9
U.S. corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities available for sale, amortized cost	3,253.0	3,581.2
Fixed maturity securities available for sale, allowances for credit losses	0.0	0.0
Fixed maturity securities, gross unrealized gains	9.9	235.9
Fixed maturity securities, gross unrealized losses	(374.1)	(14.0)
Fixed maturity securities, fair value	2,888.8	3,803.1
Foreign corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities available for sale, amortized cost	1,297.4	1,205.3
Fixed maturity securities available for sale, allowances for credit losses	0.0	0.0
Fixed maturity securities, gross unrealized gains	1.9	46.0
Fixed maturity securities, gross unrealized losses	(168.9)	(8.9)
Fixed maturity securities, fair value	$ 1,130.4	$ 1,242.4

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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