Losses, Claims and Settlement Expenses - Consolidated (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023		Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Supplementary Insurance Information, by Segment
Loss and loss adjustment expense reserves	$ 12,538.2		$ 12,221.5	$ 11,425.5	$ 10,671.0
Reinsurance Recoverables	4,977.7		4,699.5	4,125.3	3,650.5
Liability for Unpaid Claims and Claims Adjustment Expense, Net			7,521.9	7,300.2	7,020.4
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Current Year	2,886.8		2,711.1	2,610.4
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years	(305.8)		(282.6)	(210.6)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	2,581.0		2,428.4	2,399.7
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	949.8		852.7	810.9
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	1,537.7		1,354.0	1,309.0
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, excluding the reclassification of Consumer Credit Indemnity coverage	2,487.6		2,206.7	2,120.0
Liability for Unpaid Claims and Claims Adjustment Expense, Net of Reinsurance Losses Recoverable	$ 7,560.4		$ 7,521.9	$ 7,300.2
One year development of consolidated reserves	4.10%		3.90%	3.00%
Three year average annual development of consolidated reserves	3.60%
Disposal Group, Not Discontinued Operations
Supplementary Insurance Information, by Segment
Loss and loss adjustment expense reserves	$ 54.9
Liability for Unpaid Claims and Claims Adjustment Expense, Net of Reinsurance Losses Recoverable	54.9	[1]	$ 0.0	$ 0.0
General Insurance Segment [Member]
Supplementary Insurance Information, by Segment
Liability for Unpaid Claims and Claims Adjustment Expense, Net			6,824.8	6,587.0	6,328.0
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Current Year	2,770.7		2,545.1	2,418.3
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years	(234.0)		(193.1)	(137.9)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	930.6		834.4	781.5
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	1,475.6		1,279.8	1,239.8
Liability for Unpaid Claims and Claims Adjustment Expense, Net of Reinsurance Losses Recoverable	6,955.2		6,824.8	6,587.0
RFIG Run-off [Member]
Supplementary Insurance Information, by Segment
Liability for Unpaid Claims and Claims Adjustment Expense, Net			77.9	111.2	127.6
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Current Year	13.7		17.5	19.3
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years	(24.7)		(35.1)	(21.1)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	0.0		0.2	0.2
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	11.9		15.5	14.3
Liability for Unpaid Claims and Claims Adjustment Expense, Net of Reinsurance Losses Recoverable	0.0		77.9	111.2
Title Insurance Group [Member]
Supplementary Insurance Information, by Segment
Liability for Unpaid Claims and Claims Adjustment Expense, Net			612.8	594.2	556.1
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Current Year	93.6		139.6	160.6
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years	(44.9)		(50.4)	(47.6)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	14.4		13.1	21.4
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	48.7		57.3	53.4
Liability for Unpaid Claims and Claims Adjustment Expense, Net of Reinsurance Losses Recoverable	598.5		612.8	594.2
Corporate & Other [Member]
Supplementary Insurance Information, by Segment
Liability for Unpaid Claims and Claims Adjustment Expense, Net			6.3	7.6	$ 8.6
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Current Year	8.6		8.7	12.0
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years	(2.1)		(3.9)	(3.9)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	4.7		4.8	7.7
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	1.3		1.3	1.3
Liability for Unpaid Claims and Claims Adjustment Expense, Net of Reinsurance Losses Recoverable	$ 6.6		$ 6.3	$ 7.6

[1] RFIG Run-off reserves have been reclassified as held-for-sale as of December 31, 2023. See Note 2 for further discussion.