Commitments and Contingent Liabilities Reinsurance and Retention Limits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reinsurance Premiums for Insurance Companies, by Product Segment
Reinsurance Unrecoverable Amount	$ 0	$ 0
Direct Premiums Earned	6,936,700,000	6,528,400,000	$ 6,296,600,000
Assumed Premiums Earned	57,400,000	54,600,000	48,100,000
Ceded Premiums Earned	1,495,800,000	1,329,700,000	1,264,500,000
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	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	53.00%
Reinsurance Recoverable, Concentration Risk Largest Insurer, Percentage	10.10%
Reinsurance Recoverable, Concentration Risk Ten Largest Insurers, Percentage of A Rating or Better	58.30%
Reinsurance Recoverable, Concentration Risk Ten Largest Insurers, Percentage From Industry-Wide Risk Pools	5.60%
Reinsurance Recoverable, Concentration Risk Ten Largest Insurers, Percentage From Foreign Unrated Companies	25.20%
Reinsurance Recoverable, Concentration Risk Ten Largest Insurers, Percentage From Domestic Unrated Companies	10.90%
Direct Premiums Written	$ 4,966,400,000	4,673,400,000	4,422,400,000
Assumed Premiums Written	66,900,000	58,100,000	59,000,000.0
Ceded Premiums Written	1,564,300,000	1,351,100,000	1,236,200,000
Direct Premiums Earned	4,857,000,000.0	4,534,800,000	4,307,500,000
Assumed Premiums Earned	56,400,000	55,800,000	49,700,000
Ceded Premiums Earned	1,481,100,000	1,313,500,000	1,246,400,000
Reinsurance Effect on Claims and Benefits Incurred, Amount Ceded	910,200,000	745,000,000.0	728,000,000.0
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	57,500,000	73,900,000	119,000,000.0
Assumed Premiums Written	0	0	0
Ceded Premiums Written	0	0	500,000
Direct Premiums Earned	59,200,000	75,900,000	123,500,000
Assumed Premiums Earned	0	0	0
Ceded Premiums Earned	0	0	500,000
Reinsurance Effect on Claims and Benefits Incurred, Amount Ceded	0	0	0
Insurance in force, direct	10,156,800,000	13,100,500,000	16,543,900,000
Insurance in force, assumed	0	0	0
Insurance in force, ceded	0	0	2,400,000
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	1,991,300,000	1,883,300,000	1,825,700,000
Assumed Premiums Earned	1,900,000	2,300,000	2,000,000.0
Ceded Premiums Earned	$ 0	$ 0	$ 200,000