Summary of Significant Accounting Policies Losses, Claims and Settlement Expenses - Consolidated (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Sep. 30, 2014	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Supplementary Insurance Information, by Segment
Losses, claims, and settlement expenses			$ 10,671.0	$ 9,929.5	$ 9,471.2	$ 9,237.6
Liability for Unpaid Claims and Claims Adjustment Expense, Net				6,679.7	6,464.9	6,316.4
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Current Year			2,557.8	2,577.6	2,520.7
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years			(83.8)	(30.9)	(77.8)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	[1]		2,474.0	2,546.6	2,442.9
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year			795.5	851.5	842.2
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years			1,337.7	1,480.2	1,452.2
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, excluding the reclassification of Consumer Credit Indemnity coverage			2,133.2	2,331.7	2,294.5
Liability for Unpaid Claims and Claims Adjustment Expense, Net of Reinsurance Losses Recoverable			$ 7,020.4	$ 6,679.7	$ 6,464.9
One year development of consolidated reserves			1.30%	0.50%	1.20%
Three year average annual development of consolidated reserves			1.00%
General Insurance Segment [Member]
Supplementary Insurance Information, by Segment
Liability for Unpaid Claims and Claims Adjustment Expense, Net				$ 6,021.3	$ 5,766.1	5,471.5
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Current Year			$ 2,380.5	2,422.7	2,346.2
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years			(27.4)	14.5	(0.2)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year			783.2	835.4	813.2
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years			1,263.1	1,346.6	1,238.1
Liability for Unpaid Claims and Claims Adjustment Expense, Net of Reinsurance Losses Recoverable			6,328.0	6,021.3	5,766.1
RFIG Run-off Business [Member]
Supplementary Insurance Information, by Segment
Losses, claims, and settlement expenses			1.2	1.6	3.2	19.0
Liability for Unpaid Claims and Claims Adjustment Expense, Net				118.9	154.5	271.7
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Current Year	[1]		48.8	41.1	56.2
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years	[1]		(11.9)	(9.4)	(26.2)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year			1.1	3.3	3.7
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years			27.0	64.0	143.3
Liability for Unpaid Claims and Claims Adjustment Expense, Net of Reinsurance Losses Recoverable			127.6	118.9	154.5
Increase (Decrease) in Loss and Loss Adjustment Expense Reserve, Current Year			(0.4)	(0.6)	(0.9)
Increase (Decrease) in Loss and Loss Adjustment Expense Reserve, Prior Year			0.0	0.9	12.3
Deferred Payment Obligation		$ 657.0
Title Insurance Group [Member]
Supplementary Insurance Information, by Segment
Liability for Unpaid Claims and Claims Adjustment Expense, Net				530.9	533.4	559.7
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Current Year			117.2	99.5	96.1
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years			(41.8)	(32.1)	(47.7)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year			4.6	3.6	9.1
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years			45.4	66.2	65.4
Liability for Unpaid Claims and Claims Adjustment Expense, Net of Reinsurance Losses Recoverable			556.1	530.9	533.4
Corporate & Other [Member]
Supplementary Insurance Information, by Segment
Liability for Unpaid Claims and Claims Adjustment Expense, Net				8.4	10.8	13.5
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Current Year			11.2	14.1	22.1
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years			(2.5)	(3.9)	(3.5)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year			6.4	9.1	16.0
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years			2.0	3.3	5.2
Liability for Unpaid Claims and Claims Adjustment Expense, Net of Reinsurance Losses Recoverable			8.6	8.4	10.8
Reinsurance Recoverable Including Reinsurance Premium Paid [Member]
Supplementary Insurance Information, by Segment
Reinsurance Recoverables			$ 3,650.5	$ 3,249.7	$ 3,006.3	$ 2,921.1

[1] In common with all other insurance coverages, RFIG Run-off settled and incurred claim and claim adjustment expenses include only those costs actually or expected to be paid by the Company. Changes in mortgage guaranty aggregate case, IBNR, and loss adjustment expense reserves entering into the determination of incurred claim costs, take into account, among a large number of variables, claim cost reductions for anticipated coverage rescissions and claims denials. Estimates of coverage rescissions and claim denials are no longer material to Old Republic's consolidated financial statements.