Employee Benefit Plans - Schedule of Benefits Measured at Fair Value (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Funded Status of Plan
Defined benefit plan, fair value of plan assets	$ 10,811	$ 10,030
Investments measured at net asset value (NAV)	5,724	5,364
Benefit plan assets	5,087	4,666
Level 1
Defined Benefit Plan, Funded Status of Plan
Benefit plan assets	2,643	2,083
Level 2
Defined Benefit Plan, Funded Status of Plan
Benefit plan assets	2,444	2,583
Equities and equity-related instruments
Defined Benefit Plan, Funded Status of Plan
Defined benefit plan, fair value of plan assets	2,421	2,116
Investments measured at net asset value (NAV)	1,214	1,133
Equities and equity-related instruments | Level 1
Defined Benefit Plan, Funded Status of Plan
Defined benefit plan, fair value of plan assets	2,407	2,067
Equities and equity-related instruments | Level 2
Defined Benefit Plan, Funded Status of Plan
Defined benefit plan, fair value of plan assets	14	49
Cash equivalents
Defined Benefit Plan, Funded Status of Plan
Defined benefit plan, fair value of plan assets	1,468	1,700
Cash equivalents | Level 1
Defined Benefit Plan, Funded Status of Plan
Defined benefit plan, fair value of plan assets	228	0
Cash equivalents | Level 2
Defined Benefit Plan, Funded Status of Plan
Defined benefit plan, fair value of plan assets	1,240	1,700
Fixed income and fixed income-related instruments
Defined Benefit Plan, Funded Status of Plan
Defined benefit plan, fair value of plan assets	1,198	850
Investments measured at net asset value (NAV)	270	268
Fixed income and fixed income-related instruments | Level 1
Defined Benefit Plan, Funded Status of Plan
Defined benefit plan, fair value of plan assets	8	16
Fixed income and fixed income-related instruments | Level 2
Defined Benefit Plan, Funded Status of Plan
Defined benefit plan, fair value of plan assets	$ 1,190	$ 834