Acquisitions - Purchase Price Allocation (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Business Acquisition [Line Items]
Cash paid for acquisition of businesses	$ 22.0	$ 88.1
Acquisitions 2017
Business Acquisition [Line Items]
Cash paid for acquisition of businesses		87.1
Amounts due to sellers		0.5
Non-monetary consideration		8.1
Estimated purchase price		95.7
Assets acquired:
Property and equipment		10.7
Goodwill and identifiable intangible assets		85.0
Other current and long-term assets		7.6
Liabilities assumed:
Long-term liabilities and deferred tax liabilities		(7.7)
Estimated purchase price		$ 95.7