Consolidated Quarterly Results - Unaudited (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Quarterly Financial Information Disclosure [Abstract]
Net premiums, fees, and other income	$ 1,785.6	$ 1,801.1	$ 1,689.3	$ 1,594.6	$ 1,866.9	$ 1,983.1	$ 2,020.0	$ 1,955.2
Net investment income and realized gains (losses)	155.4	(40.9)	108.9	164.0	478.6	(262.0)	(209.6)	251.3
Total revenues	1,941.1	1,760.1	1,798.3	1,758.7	2,345.5	1,721.0	1,810.3	2,206.6	$ 7,258.3	$ 8,083.7	$ 9,341.6
Total expenses	1,703.6	1,696.2	1,601.4	1,509.5	1,696.5	1,840.7	1,865.0	1,823.9	6,510.8	7,226.3	7,419.5
Net income	$ 190.6	$ 52.6	$ 155.5	$ 199.8	$ 512.1	$ (91.7)	$ (40.1)	$ 306.3	$ 598.6	$ 686.4	$ 1,534.3
Net income per share: basic (in dollars per share)	$ 0.70	$ 0.19	$ 0.55	$ 0.68	$ 1.74	$ (0.31)	$ (0.13)	$ 1.01	$ 2.12	$ 2.28	$ 5.08
Net income per share: diluted (in dollars per share)	$ 0.69	$ 0.19	$ 0.54	$ 0.68	$ 1.73	$ (0.31)	$ (0.13)	$ 1.00	$ 2.10	$ 2.26	$ 5.05
Weighted average number of shares outstanding, basic	274,036,118	277,010,690	285,426,801	291,945,750	294,290,072	303,652,802	303,793,432	303,582,578	282,732,526	301,676,941	301,945,319
Weighted Average Number of Shares Outstanding, Diluted	277,226,628	279,924,410	287,882,787	293,993,474	295,996,057	303,652,802	303,793,432	305,424,592	285,471,064	303,296,612	303,667,669