Losses, Claims and Settlement Expenses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Liability for Unpaid Claims and Claims Adjustment Expense
Gross reserves at beginning of period	$ 9,206.0	$ 9,120.2
Net reserves at beginning of period			$ 6,439.8	$ 6,387.6
Provisions for insured events of the current year	574.6	589.7
Change in provision for insured events of prior years	(15.1)	(19.4)
Total incurred claims and claim adjustment expenses	559.5	570.3
Claims and claim adjustment expenses attributable to insured events of the current year	97.1	96.3
Claims and claim adjustment expenses attributable to insured events of prior years	437.8	461.1
Total payments	535.0	557.5
Amount of reserves for unpaid claims and claim adjustment expenses at the end of each period, net of reinsurance losses recoverable	6,464.3	6,400.4
Gross reserves at end of period	9,231.3	9,067.5
Net reserve increase (decrease)	24.4	12.7
Net reserves for claims that have been incurred but are not yet reported	3,250.6	3,120.3	3,160.5
Reinsurance losses recoverable
Supplementary Insurance Information, by Segment [Line Items]
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments			2,766.1	2,732.5
Liability for Unpaid Claims and Claims Adjustment Expense
Less: reinsurance losses recoverable	2,766.9	2,667.1
General Insurance
Liability for Unpaid Claims and Claims Adjustment Expense
Net reserves at beginning of period			5,249.9	5,053.1
Provisions for insured events of the current year	513.3	518.7
Change in provision for insured events of prior years	10.1	1.9
Claims and claim adjustment expenses attributable to insured events of the current year	94.1	93.5
Claims and claim adjustment expenses attributable to insured events of prior years	367.5	379.3
Amount of reserves for unpaid claims and claim adjustment expenses at the end of each period, net of reinsurance losses recoverable	5,311.7	5,100.8
Net reserve increase (decrease)	61.8	47.7
Net reserves for claims that have been incurred but are not yet reported	2,515.3	2,406.7	2,431.2
Title Insurance
Liability for Unpaid Claims and Claims Adjustment Expense
Net reserves at beginning of period			602.0	580.8
Provisions for insured events of the current year	21.3	24.4
Change in provision for insured events of prior years	(10.3)	0.0
Claims and claim adjustment expenses attributable to insured events of the current year	0.2	0.1
Claims and claim adjustment expenses attributable to insured events of prior years	14.7	17.4
Amount of reserves for unpaid claims and claim adjustment expenses at the end of each period, net of reinsurance losses recoverable	598.1	587.6
Net reserve increase (decrease)	(3.9)	6.8
Net reserves for claims that have been incurred but are not yet reported	521.5	516.5	517.5
RFIG Run-off
Liability for Unpaid Claims and Claims Adjustment Expense
Net reserves at beginning of period			574.0	736.7
Provisions for insured events of the current year	32.8	40.6
Change in provision for insured events of prior years	(13.0)	(20.4)
Claims and claim adjustment expenses attributable to insured events of the current year	0.2	0.0
Claims and claim adjustment expenses attributable to insured events of prior years	53.4	59.4
Amount of reserves for unpaid claims and claim adjustment expenses at the end of each period, net of reinsurance losses recoverable	540.2	697.3
Current year estimated coverage rescissions and claims denials	1.7	2.9
Prior period estimated coverage rescissions and claims denials	(1.6)	(0.4)
Net reserve increase (decrease)	(33.7)	(39.3)
Estimated reduction in paid losses from rescissions	(3.0)	(6.4)
Net reserves for claims that have been incurred but are not yet reported	208.6	192.1	206.3
Other
Liability for Unpaid Claims and Claims Adjustment Expense
Net reserves at beginning of period			13.8	$ 16.9
Provisions for insured events of the current year	7.0	6.0
Change in provision for insured events of prior years	(1.8)	(0.9)
Claims and claim adjustment expenses attributable to insured events of the current year	2.5	2.5
Claims and claim adjustment expenses attributable to insured events of prior years	2.2	4.9
Amount of reserves for unpaid claims and claim adjustment expenses at the end of each period, net of reinsurance losses recoverable	14.2	14.5
Net reserve increase (decrease)	0.3	(2.3)
Net reserves for claims that have been incurred but are not yet reported	$ 5.0	$ 4.8	$ 5.4