Acquisition - Narrative (Details) - Teachers On Call, Inc. - USD ($) $ in Millions	Sep. 05, 2017	Dec. 31, 2017
Business Acquisition [Line Items]
Ownership percentage acquired	100.00%
Purchase price of acquisition	$ 41.0
Purchase price paid at closing	39.0
Intangibles	18.3
Finite-lived intangibles	13.5
Goodwill expected to be deductible	18.3
Indemnification asset recognized as of the acquisition date	2.8
Indemnification asset		$ 0.5
Trademarks
Business Acquisition [Line Items]
Indefinite-lived intangibles	4.8
Customer relationships
Business Acquisition [Line Items]
Finite-lived intangibles	$ 12.0
Useful life of intangible assets acquired	10 years
Candidate database
Business Acquisition [Line Items]
Finite-lived intangibles	$ 1.5
Useful life of intangible assets acquired	4 years