Investments - Net Realized Capital Gains and Losses (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Major components of net realized capital gains (losses) on investments
Fixed maturities, available-for-sale: Gross gains		$ 8.4	$ 20.0	$ 49.1	$ 26.0
Fixed maturities, available-for-sale: Gross losses		(17.7)	(50.7)	(92.5)	(163.3)
Fixed maturities, available-for-sale: Net credit recoveries (losses)		1.0	4.3	(19.8)	(5.8)
Fixed maturities, available-for-sale: Hedging, net		(1.7)		(2.3)	(0.7)
Fixed maturities, trading		(6.4)	(12.3)	(11.6)	(21.8)
Equity securities		29.6	(37.1)	32.3	(187.5)
Mortgage loans gains (losses)		(8.1)	(2.5)	(82.4)	(19.3)
Derivative net realized capital gains (losses)		91.2	74.2	54.8	178.7
Other gains (losses)		83.0	(23.3)	112.9	(159.7)
Net realized capital gains (losses)	[1]	179.3	(27.4)	40.5	(353.4)
Unrealized gains (losses) on fixed maturities, trading		(6.6)	(5.8)	(13.0)	(12.0)
Unrealized gains (losses) on fixed maturities, trading, reported in net realized capital gains (losses) on funds withheld assets		(4.9)	0.0	(6.7)	0.0
Unrealized gains (losses) on fixed maturities, trading still head at reported in market risk benefit remeasurement (gain) loss		(1.1)	(4.3)	0.1	(17.3)
Unrealized gains (losses) on equity securities		33.0	(37.1)	40.6	(184.0)
Unrealized gains (losses) on equity securities reported in net investment income		(2.0)	(7.2)	4.3	(17.7)
Unrealized gains (losses) on equity securities reported in net realized capital gains (losses) on funds withheld assets		0.0	0.0	1.7	0.0
Proceeds from sales of investments
Proceeds from sales of investments in fixed maturities, available-for-sale		$ 755.4	$ 2,989.7	$ 3,852.5	$ 10,531.0

[1] Includes realized and unrealized gains (losses). See Note 3, Investments, for further details.