Debt (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Weighted average interest rate	1.30%
Long-term Debt	$ 1,547.4	$ 1,529.8
Commercial paper, maximum amount outstanding during period	321.3
Commercial paper, average outstanding amount	254.4
Line of credit, maximum borrowing capacity	1,100.0
Line of Credit Facility, Maximum Borrowing Capacity, Multicurrency Limit	100.0
Derivative Instruments, Gain (Loss) Recognized in Income, Net	2.4	3.9	$ 7.9
Current portion of long-term debt	5.0	2.0
Long-term Debt and Capital Lease Obligations	1,542.4	1,527.8
Long-term Debt, Fiscal Year Maturity [Abstract]
2018	239.3
2019	402.7
2020	1.7
2021	401.7
2022	$ 201.7
Notes Payable 3.1 Percent Due 2026 [Member]
Debt Instrument [Line Items]
Weighted average interest rate	3.10%
Long-term Debt	$ 300.0	300.0
Debt instrument, face amount	300.0
Other Debt Obligations [Member]
Debt Instrument [Line Items]
Long-term Debt	15.8	14.7
Interest-rate swap
Debt Instrument [Line Items]
Long-term Debt	(0.6)	1.3
Unamortized discounts and debt issuance costs [Member]
Debt Instrument [Line Items]
Long-term Debt	$ (6.0)	(7.5)
Commercial Paper [Member]
Debt Instrument [Line Items]
Weighted average interest rate	1.90%
Long-term Debt	$ 238.2	221.3
Notes Payable 6.8 Percent Due 2019 [Member]
Debt Instrument [Line Items]
Weighted average interest rate	6.80%
Long-term Debt	$ 400.0	400.0
Notes Payable 4.5 Percent Due 2021 [Member]
Debt Instrument [Line Items]
Weighted average interest rate	4.50%
Long-term Debt	$ 400.0	400.0
Debt instrument, face amount	$ 400.0
Fixed-rate (as a percent)	4.50%
Note Payable LIBOR plus fixed spread due 2022 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 200.0	200.0
Debt instrument, face amount	200.0
Interest-rate swap | Measured on a recurring basis | (Level 2)
Debt Instrument [Line Items]
Derivative Asset	0.0	1.3
Derivative Liability	$ 0.6	0.0
Derivatives designated as hedging instruments | Interest-rate swap
Debt Instrument [Line Items]
Number of swap agreements	4
Notional amounts of derivatives	$ 400.0
Derivative Instruments, Gain (Loss) Recognized in Income, Net	$ 3.2	$ 4.7	$ 7.2