Revenue - Contract Balances (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Contract assets	$ 1,286.9	$ 1,286.9	$ 1,755.5
Contract liabilities	3,711.9	3,711.9	3,314.2
Net contract assets (liabilities)	(2,425.0)	(2,425.0)	(1,558.7)
Contract assets, increase (decrease)		(468.6)
Contract (liabilities), (increase) decrease		(397.7)
Net contract assets (liabilities), increase (decrease)		$ (866.3)
Contract assets, increase (decrease), percent		(26.70%)
Contract (liabilities), (increase) decrease, percent		(12.00%)
Net contract assets (liabilities), increase (decrease), percent		(55.60%)
Contract assets, increase (decrease) due to acquisitions		$ 5.7
Deferred revenue, revenue recognized	$ 678.1	2,115.8
Performance obligation satisfied in previous periods		$ (66.4)	$ 0.4