Reinsurance and Retention Limits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Reinsurance Premiums for Insurance Companies, by Product Segment
Reinsurance Unrecoverable Amount	$ 0	$ 0
Direct Premiums Earned	8,846,400,000	9,563,300,000	$ 9,523,600,000
Assumed Premiums Earned	96,200,000	89,400,000	75,100,000
Ceded Premiums Earned	2,496,700,000	2,310,500,000	2,039,000,000
General Insurance Group [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment
Workers Compensation Insurance, Maximum Coverage Limit	5,200,000
Commercial Auto Liability Compensation Insurance, Maximum Coverage Limit	7,000,000.0
General Liabiltity Insurance, Maximum Coverage Limit	7,000,000.0
Executive Protection Insurance, Maximum Coverage Limit	12,800,000
Aviation Insurance, Maximum Coverage Limit	2,200,000
Property Coverages Insurance, Maximum Coverage Limit	23,100,000
Reinsurance Limits	195,000,000.0
Reinsurance Limits for Acts of Terrorism	5,000,000.0
Facultative reinsurance limit	$ 200,000,000.0
Reinsurance Recoverable, Concentration Risk Ten Largest Insurers, Percentage	59.00%
Reinsurance Recoverable, Concentration Risk Largest Insurer, Percentage	13.20%
Reinsurance Recoverable, Concentration Risk Ten Largest Insurers, Percentage of A Rating or Better	50.80%
Reinsurance Recoverable, Concentration Risk Ten Largest Insurers, Percentage From Foreign Unrated Companies	26.80%
Reinsurance Recoverable, Concentration Risk Ten Largest Insurers, Percentage From Domestic Unrated Companies	22.40%
Direct Premiums Written	$ 6,776,400,000	6,263,300,000	5,691,300,000
Assumed Premiums Written	96,600,000	90,000,000.0	74,000,000.0
Ceded Premiums Written	2,516,700,000	2,375,100,000	2,084,400,000
Direct Premiums Earned	6,513,200,000	6,021,000,000	5,509,100,000
Assumed Premiums Earned	94,700,000	87,100,000	73,700,000
Ceded Premiums Earned	2,488,600,000	2,299,500,000	2,027,300,000
Reinsurance Effect on Claims and Benefits Incurred, Amount Ceded	1,795,900,000	1,677,300,000	1,255,600,000
RFIG Run-off [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment
Average Direct Primary Mortgage Guaranty Exposure, Per Insured Loan	37,000
Direct Premiums Earned	16,400,000	23,200,000	32,600,000
Assumed Premiums Earned	0	0	0
Ceded Premiums Earned	0	0	0
Title Insurance Group [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment
Title Insurance Risk, Per Policy	500,000,000.0
Title Insurance Risk Exposure, Per Policy	1,000,000.0
Direct Premiums Earned	2,299,100,000	3,498,100,000	3,958,600,000
Assumed Premiums Earned	1,800,000	2,600,000	1,800,000
Ceded Premiums Earned	$ 0	$ 100,000	$ 0