Business Combinations, Intangible Assets Acquired (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Intangible Assets Acquired [Abstract]
Aggregate acquisitions	$ 20.5	$ 3.1
Customer Relationships [Member]
Intangible Assets Acquired [Abstract]
Weighted average useful life	6 years	5 years
Useful life	6 years	5 years
Aggregate acquisitions	$ 17.3	$ 2.5
Customer Backlog [Member]
Intangible Assets Acquired [Abstract]
Weighted average useful life	1 year	6 months
Useful life		6 months
Aggregate acquisitions	$ 1.8	$ 0.4
Customer Backlog [Member] | Minimum [Member]
Intangible Assets Acquired [Abstract]
Useful life	3 months
Customer Backlog [Member] | Maximum [Member]
Intangible Assets Acquired [Abstract]
Useful life	1 year
Non-Compete Agreements [Member]
Intangible Assets Acquired [Abstract]
Weighted average useful life	5 years	5 years
Useful life		5 years
Aggregate acquisitions	$ 0.7	$ 0.2
Non-Compete Agreements [Member] | Minimum [Member]
Intangible Assets Acquired [Abstract]
Useful life	2 years
Non-Compete Agreements [Member] | Maximum [Member]
Intangible Assets Acquired [Abstract]
Useful life	5 years
Trade Name [Member]
Intangible Assets Acquired [Abstract]
Weighted average useful life	1 year	1 year
Useful life	1 year	1 year
Aggregate acquisitions	$ 0.1	$ 0.0
Internally Developed Software [Member]
Intangible Assets Acquired [Abstract]
Weighted average useful life	4 years
Useful life	4 years
Aggregate acquisitions	$ 0.6