Employee Benefits - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit expense	$ 15	$ 16	$ 46
Contributions by employer	$ 146	132	$ 130
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Number of eligible employees for pension plan (in employee)	0
Defined benefit obligation	$ 1,320	1,230
Defined benefit underfunded status	9	32
Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit obligation	16	21
Defined benefit underfunded status	16	21
SERP
Defined Benefit Plan Disclosure [Line Items]
Defined benefit obligation	125	136
Defined benefit underfunded status	$ 125	$ 136