Fair Value Measurements and Risk - Gain (Loss) on Derivative Instruments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Gains and losses on derivative instruments
Interest expense and related financing costs	$ 19.9	$ 20.1	$ 39.2	$ 39.6
Other income (expense)	5.8	(11.4)	3.5	(13.3)
Cash Flow Hedging
Gains and losses on derivative instruments
Amount of Gain (Loss) Recognized in OCI on Derivatives	32.1	(1.1)	27.0	(1.1)
Amount of Gain (Loss) Reclassified from OCI into Income	42.7	(0.9)	36.4	(1.8)
Cash Flow Hedging | Interest rate derivatives
Gains and losses on derivative instruments
Amount of Gain (Loss) Recognized in OCI on Derivatives	0.3	(1.1)	1.8	(1.0)
Cash Flow Hedging | Interest rate derivatives | Interest expense
Gains and losses on derivative instruments
Amount of Gain (Loss) Reclassified from OCI into Income	0.1	(0.8)	(0.1)	(1.7)
Cash Flow Hedging | Currency contracts
Gains and losses on derivative instruments
Amount of Gain (Loss) Recognized in OCI on Derivatives	10.4	0.0	3.8	(0.1)
Derivative instruments, gain (loss) recognized in OCI, amount excluded from effectiveness testing, net	0.4		(0.4)
Cash Flow Hedging | Currency contracts | Other expense
Gains and losses on derivative instruments
Amount of Gain (Loss) Reclassified from OCI into Income	10.2	(0.1)	4.1	(0.1)
Amount excluded from effectiveness testing recognized in earnings based on amortization approach (included in above)	0.3	$ 0.0	1.1	$ 0.0
Cash Flow Hedging | Variable-to-fixed cross-currency derivatives | Interest expense
Gains and losses on derivative instruments
Amount of Gain (Loss) Recognized in OCI on Derivatives	3.1		3.1
Amount of Gain (Loss) Reclassified from OCI into Income	3.1		3.1
Cash Flow Hedging | Variable-to-fixed cross-currency derivatives | Other expense
Gains and losses on derivative instruments
Amount of Gain (Loss) Recognized in OCI on Derivatives	18.3		18.3
Amount of Gain (Loss) Reclassified from OCI into Income	$ 29.3		$ 29.3