Acquisitions (Acquisitions Costs, by Acquisition) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended	0 Months Ended	12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	May 15, 2012 Apex Systems	Dec. 31, 2012 Apex Systems
Business Acquisition [Line Items]
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Financial Assets	$ 2.2	$ 1.7
Effective Date of Acquisition				May 15, 2012
Name of Acquired Entity				Apex Systems, Inc.
Business Acquisition, Description of Acquired Entity				a privately-owned provider of information technology staffing services headquartered in Richmond, Virginia
Reason for Business Combination				The primary reason for the acquisition was to expand the Company's information technology staffing services.
Cost of acquired entity, purchase price				610.8
Cost of acquired entity, cash paid				385.3
Shares of Company's common stock issued (in shares)				14.3
Acquisition costs expensed					9.8
Purchase Price Allocation, Methodology			Assets and liabilities of the acquired company were recorded at their estimated fair values at the dates of acquisition. The excess purchase price over the fair value of net tangible assets and identifiable intangible assets acquired has been allocated to goodwill.
Purchase Price Allocation, Status	the Company finalized the allocation of the purchase price
Business Combination Provisional Information Initial Accounting Incomplete Adjustment, Financial Assets, Net of Taxes		$ 1.0