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Accounting Standards Update (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense	$ 4.5	$ 2.7	$ 6.6
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Diluted Earnings Per Share	$ 0.09
Excess Tax Benefit from Share-based Compensation, Operating Activities		$ 3.1	$ 6.6
Allowance for fallouts	$ 1.5

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allowance for fallouts permanent placement candidates that do not remain with the client through the contingency period

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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Amount of cash outflow for realized tax benefit related to deductible compensation cost reported on the entity's tax return for equity instruments in excess of the compensation cost for those instruments recognized for financial reporting purposes.

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Amount of the effect of a change in accounting principle on diluted earnings per share.

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