Financial Information for Issuer, Parent Guarantor, Other Guarantor Subsidiaries and Non-Guarantor Subsidiaries (Comprehensive Income Disclosures) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Comprehensive (loss) income	$ (6)	$ 534	$ (539)
Less: Comprehensive income attributable to noncontrolling interests	(5)	(12)	(13)
Comprehensive income attributable to Willis Group Holdings	(11)	522	(552)
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Comprehensive (loss) income	(11)	522	(552)
Less: Comprehensive income attributable to noncontrolling interests	0	0	0
Comprehensive income attributable to Willis Group Holdings	(11)	522	(552)
Guarantor Companies [Member]
Condensed Financial Statements, Captions [Line Items]
Comprehensive (loss) income	69	565	(494)
Less: Comprehensive income attributable to noncontrolling interests	0	0	0
Comprehensive income attributable to Willis Group Holdings	69	565	(494)
Trinity Acquisition Limited [Member]
Condensed Financial Statements, Captions [Line Items]
Comprehensive (loss) income	(5)	504	(528)
Less: Comprehensive income attributable to noncontrolling interests	0	0	0
Comprehensive income attributable to Willis Group Holdings	(5)	504	(528)
Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Comprehensive (loss) income	49	636	(519)
Less: Comprehensive income attributable to noncontrolling interests	(5)	(12)	(13)
Comprehensive income attributable to Willis Group Holdings	44	624	(532)
Consolidation entries [Member]
Condensed Financial Statements, Captions [Line Items]
Comprehensive (loss) income	(108)	(1,693)	1,554
Less: Comprehensive income attributable to noncontrolling interests	0	0	0
Comprehensive income attributable to Willis Group Holdings	$ (108)	$ (1,693)	$ 1,554

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Other Comprehensive Income

-URI http://asc.fasb.org/extlink&oid=6519514

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Comprehensive Income

-URI http://asc.fasb.org/extlink&oid=35737396

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 220

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e557-108580

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-Name Accounting Standards Codification

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Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income (loss) and other comprehensive income (loss), attributable to noncontrolling interests. Excludes changes in equity resulting from investments by owners and distributions to owners.

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