Employee Benefits - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) employee	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit expense	$ 16	$ 46	$ 32
Contributions by employer	$ 132	130	$ 147
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Number of eligible employees for pension plan (in employee) | employee	0
Defined benefit obligation	$ 1,230	1,180
Defined benefit underfunded status	32	16
Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit obligation	136	155
Defined benefit underfunded status	21	30
SERP
Defined Benefit Plan Disclosure [Line Items]
Defined benefit obligation	21	30
Defined benefit underfunded status	$ 136	$ 155