Summary of Significant Accounting Policies Accounting Principles (Details) - USD ($) $ 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
Total Common Shareholders' Equity		$ 6,410.7				$ 6,173.2				$ 6,410.7	$ 6,173.2
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
General Insurance Segment [Member]
Risk based capital		609.00%				654.00%				609.00%	654.00%
Costs and Expenses [Member] | Significant Reconciling Items [Member]
Total statutory net income (loss)										$ 684.3	$ 860.8	990.4
Costs and Expenses [Member] | Deferred Policy Acquisition Costs [Member]
Statutory net income amount										34.9	26.3	9.4
Costs and Expenses [Member] | Investment adjustment [Member]
Statutory net income amount										(109.1)	(252.4)	606.6
Costs and Expenses [Member] | Non-admitted Assets [Member]
Statutory net income amount										0.0	0.0	0.0
Costs and Expenses [Member] | Deferred Income Taxes [Member]
Statutory net income amount										26.2	35.4	(135.3)
Costs and Expenses [Member] | Mortgage Contingency Reserves [Member]
Statutory net income amount										0.0	0.0	0.0
Costs and Expenses [Member] | Title insurance premium reserves [Member]
Statutory net income amount										(43.7)	42.5	109.4
Costs and Expenses [Member] | Losses, claims and settlement expenses [Member]
Statutory net income amount										17.1	(25.2)	(48.7)
Costs and Expenses [Member] | Surplus Notes [Member]
Statutory net income amount										0.0	0.0	0.0
Costs and Expenses [Member] | Sundry Adjustments [Member]
Statutory net income amount										(11.2)	(0.7)	2.4
Costs and Expenses [Member] | Adjustment to US-GAAP
Statutory net income amount										(85.7)	(174.4)	543.6
Stockholders Equity [Member] | Significant Reconciling Items [Member]
Total statutory capital		$ 6,527.9				$ 6,729.0				6,527.9	6,729.0
Stockholders Equity [Member] | Deferred Policy Acquisition Costs [Member]
Statutory capital and surplus, balance		286.7				252.9				286.7	252.9
Stockholders Equity [Member] | Investment adjustment [Member]
Statutory capital and surplus, balance		(102.6)				(537.6)				(102.6)	(537.6)
Stockholders Equity [Member] | Non-admitted Assets [Member]
Statutory capital and surplus, balance		207.3				173.9				207.3	173.9
Stockholders Equity [Member] | Deferred Income Taxes [Member]
Statutory capital and surplus, balance		(95.1)				4.0				(95.1)	4.0
Stockholders Equity [Member] | Mortgage Contingency Reserves [Member]
Statutory capital and surplus, balance		38.4				127.7				38.4	127.7
Stockholders Equity [Member] | Title insurance premium reserves [Member]
Statutory capital and surplus, balance		733.7				777.5				733.7	777.5
Stockholders Equity [Member] | Losses, claims and settlement expenses [Member]
Statutory capital and surplus, balance		(535.8)				(548.8)				(535.8)	(548.8)
Stockholders Equity [Member] | Surplus Notes [Member]
Statutory capital and surplus, balance		(696.5)				(844.5)				(696.5)	(844.5)
Stockholders Equity [Member] | Sundry Adjustments [Member]
Statutory capital and surplus, balance		46.4				38.9				46.4	38.9
Stockholders Equity [Member] | Adjustment to US-GAAP
Statutory capital and surplus, balance		(117.5)				(556.2)				(117.5)	(556.2)
Insurance Subsidiaries [Member] | Costs and Expenses [Member]
Total statutory net income (loss)	[1]									761.6	849.6	813.4
Insurance Subsidiaries [Member] | Costs and Expenses [Member] | General Insurance Segment [Member]
Total statutory net income (loss)	[1]									594.3	549.2	496.8
Insurance Subsidiaries [Member] | Costs and Expenses [Member] | Title Insurance Group [Member]
Total statutory net income (loss)	[1]									152.3	224.9	285.7
Insurance Subsidiaries [Member] | Costs and Expenses [Member] | RFIG Run-off [Member]
Total statutory net income (loss)	[1]									9.8	70.5	27.3
Insurance Subsidiaries [Member] | Costs and Expenses [Member] | Life & Accident Insurance [Member]
Total statutory net income (loss)	[1]									5.2	5.0	3.6
Insurance Subsidiaries [Member] | Stockholders Equity [Member]
Total statutory capital	[1]	5,469.4				5,705.3				5,469.4	5,705.3
Insurance Subsidiaries [Member] | Stockholders Equity [Member] | General Insurance Segment [Member]
Total statutory capital	[1]	4,607.8				4,763.4				4,607.8	4,763.4
Insurance Subsidiaries [Member] | Stockholders Equity [Member] | Title Insurance Group [Member]
Total statutory capital	[1]	673.9				742.7				673.9	742.7
Insurance Subsidiaries [Member] | Stockholders Equity [Member] | RFIG Run-off [Member]
Total statutory capital	[1]	131.1				141.8				131.1	141.8
Insurance Subsidiaries [Member] | Stockholders Equity [Member] | Life & Accident Insurance [Member]
Total statutory capital	[1]	56.6				57.4				56.6	57.4
NonInsurance Subsidiaries [Member] | Costs and Expenses [Member] | Significant Reconciling Items [Member]
Total statutory net income (loss)										(77.2)	11.3	$ 177.0
NonInsurance Subsidiaries [Member] | Stockholders Equity [Member] | Significant Reconciling Items [Member]
Total statutory capital		$ 1,058.6				$ 1,023.7				$ 1,058.6	$ 1,023.7

[1] The insurance laws of the respective states in which the Company’s insurance subsidiaries are incorporated prescribe minimum capital and surplus requirements for the lines of business they are licensed to write. For domestic property and casualty and life and accident insurance companies the National Association of Insurance Commissioners also prescribes risk-based capital (RBC) requirements. RBC is a measure of statutory capital in relationship to a formula-driven definition of risk relative to a company’s balance sheet and mix of business. The combined RBC ratio of the primary General Insurance subsidiaries was 609% and 654% of the company action level RBC at December 31, 2023 and 2022, respectively. The minimum capital requirements for the Company’s Title Insurance subsidiaries are established by statute in the respective states of domicile. The minimum regulatory capital requirements are not significant in relationship to the recorded statutory capital of the Company’s Title and Life and Accident insurance subsidiaries. At December 31, 2023 and 2022 each of the Company’s General, Title, RFIG Run-off, and Life and Accident insurance subsidiaries exceeded the minimum statutory capital and surplus requirements.