Shareholders' Equity (Details 2) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 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	$ 5,247		$ 5,247		$ 5,532
Other comprehensive income (loss) before reclassifications, net			455
Amounts reclassified from accumulated other comprehensive loss:
Amounts reclassified from accumulated other comprehensive income (loss)			67
Tax benefit (expense)			(22)
Amounts reclassified from accumulated other comprehensive income (loss), net			45
Total other comprehensive income (loss) attributable to Aon shareholders	265	$ (71)	500	$ (370)
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, including portion attributable to noncontrolling interest	(3,412)		(3,412)		(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	(24)		(24)		(37)
Other comprehensive income (loss) before reclassifications, net			13
Amounts reclassified from accumulated other comprehensive loss:
Amounts reclassified from accumulated other comprehensive income (loss)			(2)
Tax benefit (expense)			2
Amounts reclassified from accumulated other comprehensive income (loss), net			0
Total other comprehensive income (loss) attributable to Aon shareholders			13
Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, including portion attributable to noncontrolling interest	(833)		(833)		(1,264)
Other comprehensive income (loss) before reclassifications, net			442
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			431
Post-Retirement Benefit Obligation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, including portion attributable to noncontrolling interest	$ (2,555)		(2,555)		$ (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)			80
Tax benefit (expense)			(24)
Amounts reclassified from accumulated other comprehensive income (loss), net			56
Total other comprehensive income (loss) attributable to Aon shareholders			$ 56