Taxation (Schedule of Deferred Tax Recognized on Balance Sheet) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets	$ 234.8	$ 328.2
Valuation allowance	(94.7)	(59.0)
Deferred tax assets, net of valuation allowance	140.1	269.2
Total deferred tax liabilities	(416.3)	(559.9)
Deferred Tax Liabilities, Net	(276.2)	(290.7)
Deferred Compensation Arrangements [Member]
Deferred tax assets	57.6	79.1
Accrued Rent Expenses [Member]
Deferred tax assets	7.2	12.3
Tax Loss Carryforwards [Member]
Deferred tax assets	94.6	64.8
Postretirement Medical, Pension and Other Benefits [Member]
Deferred tax assets	16.6	24.4
Investment Basis Differences [Member]
Deferred tax assets	32.8	58.3
Accrued bonus [Member]
Deferred tax assets	8.8	64.8
Other Deferred Tax Assets [Member]
Deferred tax assets	17.2	24.5
Deferred Sales Commissions [Member]
Total deferred tax liabilities	(11.0)	(13.5)
Goodwill and Intangibles [Member]
Total deferred tax liabilities	(377.3)	(509.6)
Revaluation Reserve [Member]
Total deferred tax liabilities	(19.2)	(27.6)
Other Deferred Tax Liability [Member]
Total deferred tax liabilities	$ (8.8)	$ (9.2)