Goodwill and Other Intangible Assets - Schedule of estimated future amortization expense on intangible assets (Details) $ in Millions	Dec. 31, 2016 USD ($)
Finite-Lived Intangible Assets [Line Items]
2017	$ 259
2018	209
2019	180
2020	158
2021	119
Thereafter	300
Total future amortization of intangible assets	1,225
Risk Solutions
Finite-Lived Intangible Assets [Line Items]
2017	123
2018	118
2019	107
2020	97
2021	66
Thereafter	239
Total future amortization of intangible assets	750
HR Solutions
Finite-Lived Intangible Assets [Line Items]
2017	136
2018	91
2019	73
2020	61
2021	53
Thereafter	61
Total future amortization of intangible assets	$ 475