Investments - Amortized Cost, Gross Unrealized Gains and Losses, Fair Value and OTTI (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, cost or amortized cost	$ 11,104.5	$ 10,834.0
Fixed maturity securities, fair value	12,420.8	11,257.1
Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, cost or amortized cost	11,104.5	10,834.0
Fixed maturity securities, gross unrealized gains	1,323.8	531.4
Fixed maturity securities, gross unrealized losses	(7.5)	(108.3)
Fixed maturity securities, fair value	12,420.8	11,257.1
Other-than-temporary impairment loss, debt securities, available-for-sale, portion in other comprehensive loss, before tax, including portion attributable to noncontrolling interest	20.0	19.1
Fixed maturity securities \| U.S. government and government agencies and authorities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, cost or amortized cost	187.7	381.4
Fixed maturity securities, gross unrealized gains	6.4	4.4
Fixed maturity securities, gross unrealized losses	(0.1)	(1.2)
Fixed maturity securities, fair value	194.0	384.6
Other-than-temporary impairment loss, debt securities, available-for-sale, portion in other comprehensive loss, before tax, including portion attributable to noncontrolling interest	0.0	0.0
Fixed maturity securities \| States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, cost or amortized cost	234.8	238.9
Fixed maturity securities, gross unrealized gains	29.6	17.6
Fixed maturity securities, gross unrealized losses	0.0	(0.3)
Fixed maturity securities, fair value	264.4	256.2
Other-than-temporary impairment loss, debt securities, available-for-sale, portion in other comprehensive loss, before tax, including portion attributable to noncontrolling interest	0.0	0.0
Fixed maturity securities \| Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, cost or amortized cost	873.5	856.3
Fixed maturity securities, gross unrealized gains	108.8	58.8
Fixed maturity securities, gross unrealized losses	(0.1)	(3.0)
Fixed maturity securities, fair value	982.2	912.1
Other-than-temporary impairment loss, debt securities, available-for-sale, portion in other comprehensive loss, before tax, including portion attributable to noncontrolling interest	0.0	0.0
Fixed maturity securities \| Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, cost or amortized cost	481.9	513.6
Fixed maturity securities, gross unrealized gains	3.0	0.5
Fixed maturity securities, gross unrealized losses	(2.6)	(9.6)
Fixed maturity securities, fair value	482.3	504.5
Other-than-temporary impairment loss, debt securities, available-for-sale, portion in other comprehensive loss, before tax, including portion attributable to noncontrolling interest	0.0	0.0
Fixed maturity securities \| Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, cost or amortized cost	226.8	79.1
Fixed maturity securities, gross unrealized gains	13.8	2.2
Fixed maturity securities, gross unrealized losses	(0.6)	(1.6)
Fixed maturity securities, fair value	240.0	79.7
Other-than-temporary impairment loss, debt securities, available-for-sale, portion in other comprehensive loss, before tax, including portion attributable to noncontrolling interest	0.0	0.0
Fixed maturity securities \| Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, cost or amortized cost	1,235.7	1,399.1
Fixed maturity securities, gross unrealized gains	61.8	21.5
Fixed maturity securities, gross unrealized losses	(1.0)	(14.8)
Fixed maturity securities, fair value	1,296.5	1,405.8
Other-than-temporary impairment loss, debt securities, available-for-sale, portion in other comprehensive loss, before tax, including portion attributable to noncontrolling interest	3.6	5.0
Fixed maturity securities \| U.S. corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, cost or amortized cost	5,757.9	5,337.0
Fixed maturity securities, gross unrealized gains	829.9	315.7
Fixed maturity securities, gross unrealized losses	(2.7)	(59.7)
Fixed maturity securities, fair value	6,585.1	5,593.0
Other-than-temporary impairment loss, debt securities, available-for-sale, portion in other comprehensive loss, before tax, including portion attributable to noncontrolling interest	16.4	14.1
Fixed maturity securities \| Foreign corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, cost or amortized cost	2,106.2	2,028.6
Fixed maturity securities, gross unrealized gains	270.5	110.7
Fixed maturity securities, gross unrealized losses	(0.4)	(18.1)
Fixed maturity securities, fair value	2,376.3	2,121.2
Other-than-temporary impairment loss, debt securities, available-for-sale, portion in other comprehensive loss, before tax, including portion attributable to noncontrolling interest	$ 0.0	$ 0.0

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, before tax, of unrealized loss 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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Amortized cost 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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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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Amount, before tax, of other-than-temporary impairment (OTTI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), recognized in other comprehensive income (OCI).

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