Goodwill and Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Goodwill And Intangible Assets [Line Items]
Goodwill, beginning of year	$ 388.1	$ 278.3
Additions	2.7	110.1
Foreign currency translation adjustment	(1.1)	(0.3)
Goodwill, end of year	389.7	388.1
Indefinite-lived intangible assets, net carrying value, beginning of year	23.9	23.9
Indefinite-lived intangible assets, net carrying value, end of year	23.9	23.9
Finite-Lived Intangible Assets, Net [Abstract]
Gross carrying value	117.8	119.9	$ 41.3
Additions	0.0	83.5
Impairments	(0.9)	(1.4)
Accumulated amortization	(37.0)	(27.2)	(18.9)
Amortization of intangible assets	(9.8)	(8.3)
Foreign currency translation adjustment	(1.2)	(3.5)
Net carrying value	80.8	92.7	22.4
Goodwill, accumulated impairment loss			$ 0.2
Intangible assets, accumulated impairment loss	$ 9.1
Expected remaining useful life	13 years
North American Insurance
Schedule Of Goodwill And Intangible Assets [Line Items]
Goodwill, end of year	$ 274.3
Global Markets Insurance
Schedule Of Goodwill And Intangible Assets [Line Items]
Goodwill, end of year	111.5
Reinsurance
Schedule Of Goodwill And Intangible Assets [Line Items]
Goodwill, end of year	3.9
Distribution channels
Finite-Lived Intangible Assets, Net [Abstract]
Net carrying value	$ 73.7	81.0
Expected remaining useful life	14 years
Customer renewals
Finite-Lived Intangible Assets, Net [Abstract]
Net carrying value	$ 7.1	$ 11.7
Expected remaining useful life	2 years 8 months 12 days