Commitments and Contingent Liabilities Reinsurance and Retention Limits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018
Reinsurance Premiums for Insurance Companies, by Product Segment
Reinsurance Unrecoverable Amount	$ 0	$ 0
Direct Premiums Earned	7,993,200,000	7,358,200,000	[1]	$ 6,926,100,000	[1]
Assumed Premiums Earned	71,500,000	57,400,000	[1]	54,600,000	[1]
Ceded Premiums Earned	1,718,900,000	1,495,800,000	[1]	1,329,700,000	[1]
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	6,400,000
General Liabiltity Insurance, Maximum Coverage Limit	6,400,000
Executive Protection Insurance, Maximum Coverage Limit	12,000,000.0
Aviation Insurance, Maximum Coverage Limit	2,000,000.0
Property Coverages Insurance, Maximum Coverage Limit	5,000,000.0
Workers Compensation, Coverage Limit	200,000,000.0
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	56.00%
Reinsurance Recoverable, Concentration Risk Largest Insurer, Percentage	10.50%
Reinsurance Recoverable, Concentration Risk Ten Largest Insurers, Percentage of A Rating or Better	57.40%
Reinsurance Recoverable, Concentration Risk Ten Largest Insurers, Percentage From Foreign Unrated Companies	25.40%
Reinsurance Recoverable, Concentration Risk Ten Largest Insurers, Percentage From Domestic Unrated Companies	17.20%
Direct Premiums Written	$ 5,206,900,000	4,966,400,000		4,673,400,000
Assumed Premiums Written	70,600,000	66,900,000		58,100,000
Ceded Premiums Written	1,846,200,000	1,564,300,000		1,351,100,000
Direct Premiums Earned	5,030,200,000	4,857,000,000.0		4,534,800,000
Assumed Premiums Earned	70,300,000	56,400,000		55,800,000
Ceded Premiums Earned	1,706,300,000	1,481,100,000		1,313,500,000
Reinsurance Effect on Claims and Benefits Incurred, Amount Ceded	1,100,700,000	910,200,000		745,000,000.0
RFIG Run-off Business [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment
Average Direct Primary Mortgage Guaranty Exposure, Per Insured Loan	37,000
Direct Premiums Written	44,300,000	57,500,000		73,900,000
Assumed Premiums Written	0	0		0
Ceded Premiums Written	0	0		0
Direct Premiums Earned	45,100,000	59,200,000		75,900,000
Assumed Premiums Earned	0	0		0
Ceded Premiums Earned	0	0		0
Reinsurance Effect on Claims and Benefits Incurred, Amount Ceded	0	0		0
Insurance in force, direct	7,883,200,000	10,156,800,000		13,100,500,000
Insurance in force, assumed	0	0		0
Insurance in force, ceded	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	2,892,800,000	2,412,800,000	[1]	2,281,000,000.0	[1]
Assumed Premiums Earned	1,600,000	1,900,000	[1]	2,300,000	[1]
Ceded Premiums Earned	$ 100,000	$ 0	[1]	$ 0	[1]

[1] Certain reclassifications were made to net premiums earned, title, escrow, and other fees, and underwriting, acquisition, and other expenses in 2019 and 2018 to conform to the presentation adopted in 2020.