Goodwill and Other Intangible Assets - Schedule of estimated future amortization expense on intangible assets (Details) $ in Millions	Dec. 31, 2015 USD ($)
Finite-Lived Intangible Assets [Line Items]
2016	$ 271
2017	226
2018	167
2019	138
2020	118
Thereafter	240
Total future amortization of intangible assets	1,160
Risk Solutions [Member]
Finite-Lived Intangible Assets [Line Items]
2016	100
2017	90
2018	76
2019	66
2020	58
Thereafter	120
Total future amortization of intangible assets	510
HR Solutions [Member]
Finite-Lived Intangible Assets [Line Items]
2016	171
2017	136
2018	91
2019	72
2020	60
Thereafter	120
Total future amortization of intangible assets	$ 650