Financial Information for Issuers and Other Guarantor Subsidiaries - Narrative (Details) € in Millions		Sep. 30, 2018 USD ($)	Sep. 10, 2018 USD ($)	May 16, 2017 USD ($)	Mar. 07, 2017 USD ($)	May 26, 2016 USD ($)	May 26, 2016 EUR (€)	Mar. 22, 2016 USD ($)	Mar. 15, 2016 USD ($)	Aug. 15, 2013 USD ($)	Sep. 29, 2009 USD ($)
Revolving $1.25 billion credit facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity			$ 1,250,000,000
Revolving Credit Facility [Member] | Revolving $1.25 billion credit facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	[1]	$ 1,250,000,000
Trinity Acquisition plc [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity					$ 1,250,000,000
Trinity Acquisition plc [Member] | Revolving Credit Facility [Member] | Revolving $1.25 billion credit facility [Member]
Debt Instrument [Line Items]
Long-term debt		230,000,000
Senior Notes [Member] | Willis North America [Member]
Debt Instrument [Line Items]
Long-term debt		1,800,000,000	$ 1,000,000,000	$ 650,000,000							$ 187,000,000
Senior Notes [Member] | Trinity Acquisition plc [Member]
Debt Instrument [Line Items]
Long-term debt		$ 2,100,000,000				$ 609,000,000	€ 540	$ 1,000,000,000		$ 525,000,000
Parent Company [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Long-term debt									$ 500,000,000

[1] Notes issued in Euro (€540 million)