Summary of Significant Accounting Policies (Effects of Adoption of Topic 606 - Unaudited Consolidated Balance Sheets) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Prepaid expenses and other current assets	$ 1,080.5	$ 795.3
Other current liabilities	1,218.4	908.8
Other non-current liabilities	2,074.1	2,197.9
Retained earnings	10,594.7	$ 9,920.6
ASU 2014-09 | Before Adoption of Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Prepaid expenses and other current assets	1,035.8
Other current liabilities	1,229.2
Other non-current liabilities	2,076.5
Retained earnings	10,564.5
ASU 2014-09 | Effect of change
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Prepaid expenses and other current assets	44.7
Other current liabilities	(10.8)
Other non-current liabilities	(2.4)
Retained earnings	$ 30.2