Acquisitions - Purchase Price Allocation (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Business Acquisition [Line Items]
Cash paid for acquisition of businesses	$ 21.0	$ 88.4
Acquisitions 2017
Business Acquisition [Line Items]
Cash paid for acquisition of businesses		87.6
Non-monetary consideration		4.3
Estimated purchase price		91.9
Assets acquired:
Property and equipment		11.2
Goodwill and identifiable intangible assets		79.4
Other current and long-term assets		8.0
Liabilities assumed:
Long-term liabilities and deferred tax liabilities		(6.7)
Estimated purchase price		$ 91.9