Acquisitions (Details) In Millions, except Per Share data, unless otherwise specified	9 Months Ended		12 Months Ended						9 Months Ended
	Sep. 30, 2011 USD ($)	Sep. 30, 2010 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Sep. 30, 2011 Proservia SA [Member] EUR (€)	Sep. 22, 2011 Proservia SA [Member]
Acquisitions [Abstract]
Amount of cash consideration paid for acquisitions	$ 32.1	$ 260.6
Estimated amortization expense, remainder of the current fiscal year								10.0
Estimated amortization expense, Year 1							35.9
Estimated amortization expense, Year 2						31.1
Estimated amortization expense, Year 3					25.9
Estimated amortization expense, Year 4				22.6
Estimated amortization expense, Year 5			$ 19.4
Business Acquisition [Line Items]
Acquisition date									2011-09-22
Shares and voting rights acquired (in hundredths)										70.00%
Acquisition cost (in Euro per share)									€ 14.89