Shareholders' Equity (Details 2) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, including portion attributable to noncontrolling interest	$ 5,553		$ 5,553		$ 5,532
Other comprehensive income (loss) before reclassifications, net			211
Amounts reclassified from accumulated other comprehensive loss:
Amounts reclassified from accumulated other comprehensive income (loss)			37
Tax benefit (expense)			(13)
Amounts reclassified from accumulated other comprehensive income (loss), net			24
Total other comprehensive income (loss) attributable to Aon shareholders	73	$ (12)	235	$ (299)
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, including portion attributable to noncontrolling interest	(3,677)		(3,677)		(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	(35)		(35)		(37)
Other comprehensive income (loss) before reclassifications, net			5
Amounts reclassified from accumulated other comprehensive loss:
Amounts reclassified from accumulated other comprehensive income (loss)			(6)
Tax benefit (expense)			3
Amounts reclassified from accumulated other comprehensive income (loss), net			(3)
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	(1,069)		(1,069)		(1,264)
Other comprehensive income (loss) before reclassifications, net			206
Amounts reclassified from accumulated other comprehensive loss:
Amounts reclassified from accumulated other comprehensive income (loss)			(11)
Tax benefit (expense)			0
Amounts reclassified from accumulated other comprehensive income (loss), net			(11)
Total other comprehensive income (loss) attributable to Aon shareholders			195
Post-Retirement Benefit Obligation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, including portion attributable to noncontrolling interest	$ (2,573)		(2,573)		$ (2,611)
Other comprehensive income (loss) before reclassifications, net			0
Amounts reclassified from accumulated other comprehensive loss:
Amounts reclassified from accumulated other comprehensive income (loss)			54
Tax benefit (expense)			(16)
Amounts reclassified from accumulated other comprehensive income (loss), net			38
Total other comprehensive income (loss) attributable to Aon shareholders			$ 38