Shareholders' Equity (Details 2) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, including portion attributable to noncontrolling interest	$ 5,826		$ 5,532
Other comprehensive (loss) income before reclassifications, net	150
Amounts reclassified from accumulated other comprehensive loss:
Amounts reclassified from accumulated other comprehensive (loss) income	16
Tax benefit (expense)	(4)
Amounts reclassified from accumulated other comprehensive (loss) income, net	12
Total other comprehensive income (loss) attributable to Aon shareholders	162	$ (287)
Ending balance	(3,750)
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, including portion attributable to noncontrolling interest	(3,750)		(3,912)
Change in Fair Value of Financial Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, including portion attributable to noncontrolling interest	(39)		(37)
Other comprehensive (loss) income before reclassifications, net	4
Amounts reclassified from accumulated other comprehensive loss:
Amounts reclassified from accumulated other comprehensive (loss) income	(10)
Tax benefit (expense)	4
Amounts reclassified from accumulated other comprehensive (loss) income, net	(6)
Total other comprehensive income (loss) attributable to Aon shareholders	(2)
Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, including portion attributable to noncontrolling interest	(1,118)		(1,264)
Other comprehensive (loss) income before reclassifications, net	146
Amounts reclassified from accumulated other comprehensive loss:
Amounts reclassified from accumulated other comprehensive (loss) income	0
Tax benefit (expense)	0
Amounts reclassified from accumulated other comprehensive (loss) income, net	0
Total other comprehensive income (loss) attributable to Aon shareholders	146
Post-Retirement Benefit Obligation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, including portion attributable to noncontrolling interest	(2,593)		$ (2,611)
Other comprehensive (loss) income before reclassifications, net	0
Amounts reclassified from accumulated other comprehensive loss:
Amounts reclassified from accumulated other comprehensive (loss) income	26
Tax benefit (expense)	(8)
Amounts reclassified from accumulated other comprehensive (loss) income, net	18
Total other comprehensive income (loss) attributable to Aon shareholders	$ 18