Shareholders' Equity (Details 2) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, including portion attributable to noncontrolling interest	$ 5,385	$ 5,826	$ 5,155	$ 4,648	$ 5,532
Adoption of new accounting guidance				492	$ 49
Other comprehensive income before reclassifications, net	277
Amounts reclassified from accumulated other comprehensive loss:
Amounts reclassified from accumulated other comprehensive income	38
Tax expense	(9)
Amounts reclassified from accumulated other comprehensive income, net	29
Total other comprehensive income attributable to Aon shareholders	306	$ 162
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, including portion attributable to noncontrolling interest	(3,191)			(3,497)
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	(12)			(26)
Other comprehensive income before reclassifications, net	11
Amounts reclassified from accumulated other comprehensive loss:
Amounts reclassified from accumulated other comprehensive income	4
Tax expense	(1)
Amounts reclassified from accumulated other comprehensive income, net	3
Total other comprehensive income attributable to Aon shareholders	14
Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, including portion attributable to noncontrolling interest	(635)			(879)
Other comprehensive income before reclassifications, net	244
Amounts reclassified from accumulated other comprehensive loss:
Amounts reclassified from accumulated other comprehensive income	0
Tax expense	0
Amounts reclassified from accumulated other comprehensive income, net	0
Total other comprehensive income attributable to Aon shareholders	244
Post-Retirement Benefit Obligation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, including portion attributable to noncontrolling interest	(2,544)			(2,592)
Other comprehensive income before reclassifications, net	22
Amounts reclassified from accumulated other comprehensive loss:
Amounts reclassified from accumulated other comprehensive income	34
Tax expense	(8)
Amounts reclassified from accumulated other comprehensive income, net	26
Total other comprehensive income attributable to Aon shareholders	$ 48
As Reported | Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, including portion attributable to noncontrolling interest				(3,496)
As Reported | 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				(25)
As Reported | Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, including portion attributable to noncontrolling interest				(879)
As Reported | Post-Retirement Benefit Obligation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, including portion attributable to noncontrolling interest				(2,592)
Accounting Standards Update 2016-01 | Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adoption of new accounting guidance				(1)
Accounting Standards Update 2016-01 | Change in Fair Value of Financial Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adoption of new accounting guidance				(1)
Accounting Standards Update 2016-01 | Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adoption of new accounting guidance				0
Accounting Standards Update 2016-01 | Post-Retirement Benefit Obligation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adoption of new accounting guidance				$ 0