Employee Benefits - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) employee	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit expense	$ 46	$ 32	$ 42
Contributions by employer	$ 130	147	$ 134
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Number of eligible employees for pension plan (in employee) | employee	0
Defined benefit obligation	$ 1,180	1,260
Defined benefit under/(over) funded status	(16)	(50)
Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit obligation	155	168
Defined benefit under/(over) funded status	(30)	(120)
SERP
Defined Benefit Plan Disclosure [Line Items]
Defined benefit obligation	30	120
Defined benefit under/(over) funded status	$ (155)	$ (168)