Losses, Claims and Settlement Expenses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Liability for Unpaid Claims and Claims Adjustment Expense
Gross reserves at beginning of period	$ 9,471.2	$ 9,237.6
Net reserves at beginning of period			$ 6,464.9	$ 6,316.4
Provisions for insured events of the current year	620.5	591.9
Change in provision for insured events of prior years	(21.3)	(1.8)
Total incurred claims and claim adjustment expenses	599.2	590.1
Claims and claim adjustment expenses attributable to insured events of the current year	102.0	106.7
Claims and claim adjustment expenses attributable to insured events of prior years	467.3	418.1
Total payments	569.3	524.8
Amount of reserves for unpaid claims and claim adjustment expenses at the end of each period, net of reinsurance losses recoverable	6,494.7	6,381.7
Gross reserves at end of period	9,488.4	9,272.4
Reinsurance losses recoverable
Supplementary Insurance Information, by Segment [Line Items]
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments			3,006.3	2,921.1
Liability for Unpaid Claims and Claims Adjustment Expense
Less: reinsurance losses recoverable	2,993.7	2,890.7
General Insurance
Liability for Unpaid Claims and Claims Adjustment Expense
Net reserves at beginning of period			5,766.1	5,471.5
Provisions for insured events of the current year	584.3	546.3
Change in provision for insured events of prior years	(11.5)	15.9
Claims and claim adjustment expenses attributable to insured events of the current year	100.5	104.2
Claims and claim adjustment expenses attributable to insured events of prior years	426.7	368.0
Amount of reserves for unpaid claims and claim adjustment expenses at the end of each period, net of reinsurance losses recoverable	5,811.6	5,561.5
Title Insurance
Liability for Unpaid Claims and Claims Adjustment Expense
Net reserves at beginning of period			533.4	559.7
Provisions for insured events of the current year	20.2	21.1
Change in provision for insured events of prior years	(5.7)	(8.2)
Claims and claim adjustment expenses attributable to insured events of the current year	0.1	0.1
Claims and claim adjustment expenses attributable to insured events of prior years	14.1	13.6
Amount of reserves for unpaid claims and claim adjustment expenses at the end of each period, net of reinsurance losses recoverable	533.6	558.7
RFIG Run-off
Liability for Unpaid Claims and Claims Adjustment Expense
Net reserves at beginning of period			154.5	271.7
Provisions for insured events of the current year	11.3	18.7
Change in provision for insured events of prior years	(2.8)	(7.2)
Claims and claim adjustment expenses attributable to insured events of the current year	0.0	0.0
Claims and claim adjustment expenses attributable to insured events of prior years	24.4	34.0
Amount of reserves for unpaid claims and claim adjustment expenses at the end of each period, net of reinsurance losses recoverable	138.5	249.1
Other
Liability for Unpaid Claims and Claims Adjustment Expense
Net reserves at beginning of period			$ 10.8	$ 13.5
Provisions for insured events of the current year	4.6	5.7
Change in provision for insured events of prior years	(1.2)	(2.3)
Claims and claim adjustment expenses attributable to insured events of the current year	1.3	2.2
Claims and claim adjustment expenses attributable to insured events of prior years	2.0	2.4
Amount of reserves for unpaid claims and claim adjustment expenses at the end of each period, net of reinsurance losses recoverable	$ 10.8	$ 12.2