Stockholders' Equity - Number and Cost of Common Stock Shares Repurchased (Details) - USD ($) shares in Thousands, $ in Thousands	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Equity [Abstract]
Common stock repurchased (in shares)	2,194	2,137
Common stock repurchased	$ 135,989	$ 101,068

This element represents the value of stock that has been repurchased during the period that is held in treasury and is presented as if constructively retired. Some state laws may mandate the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

No definition available.

Name:	rhi_CommonStockRepurchasedValue
Namespace Prefix:	rhi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration