Fair Value Measurements - Narrative (Details) - USD ($)	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Fair value significant transfers between Levels 1, 2 or 3	$ 0	$ 0

Fair value level one level two level or three transfers amount.

No definition available.

Name:	wltw_FairValueLevelOneLevelTwoOrLevelThreeTransfersAmount
Namespace Prefix:	wltw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration