Income Taxes - Components of Aon's deferred tax assets and liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets:
Employee benefit plans	$ 739	$ 623
Net operating/capital loss and tax credit carryforwards	295	354
Other accrued expenses	44	48
Investment basis differences	45us-gaap_DeferredTaxAssetsInvestments	50us-gaap_DeferredTaxAssetsInvestments
Accrued interest	303	94
Other	46us-gaap_DeferredTaxAssetsOther	58us-gaap_DeferredTaxAssetsOther
Total	1,472us-gaap_DeferredTaxAssetsGross	1,227us-gaap_DeferredTaxAssetsGross
Valuation allowance on deferred tax assets	(205)	(127)
Total	1,267us-gaap_DeferredTaxAssetsNet	1,100us-gaap_DeferredTaxAssetsNet
Deferred tax liabilities:
Intangibles and property, plant and equipment	(1,058)	(1,074)
Unremitted earnings	(28)	(51)
Deferred revenue	(28)	(27)
Other accrued expenses	(40)	(39)
Unrealized investment gains	(8)	0
Unrealized foreign exchange gains	(44)	(27)
Other	(20)us-gaap_DeferredTaxLiabilitiesOther	(64)us-gaap_DeferredTaxLiabilitiesOther
Total	(1,226)us-gaap_DeferredTaxLiabilities	(1,282)us-gaap_DeferredTaxLiabilities
Net deferred tax asset (liability)	$ 41us-gaap_DeferredTaxAssetsLiabilitiesNet	$ (182)us-gaap_DeferredTaxAssetsLiabilitiesNet

The sum of the tax effects as of the balance sheet date of the amount of (1) excess of tax deductions over gross income in a year which cannot be used on the tax returns in the current year but can be carried forward to reduce taxable income or income taxes payable in a future year, for which there must be sufficient tax-basis income to utilize a portion or all of the carryforward amount to realize the deferred tax asset, and capital losses that cannot be utilized on the tax return during a period that have been carried forward to reduce taxable income or taxes payable in a future year and (2) future tax deductions arising from all unused tax credit carryforwards which have been reduced by a valuation allowance.

No definition available.