Goodwill and Other Intangible Assets - Schedule of estimated future amortization expense on intangible assets (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]
2015	$ 322
2016	278
2017	232
2018	172
2019	143
Thereafter	354
Total future amortization of intangible assets	1,501
Risk Solutions [Member]
Finite-Lived Intangible Assets [Line Items]
2015	113
2016	103
2017	93
2018	80
2019	69
Thereafter	186
Total future amortization of intangible assets	644
HR Solutions [Member]
Finite-Lived Intangible Assets [Line Items]
2015	209
2016	175
2017	139
2018	92
2019	74
Thereafter	168
Total future amortization of intangible assets	$ 857