Retirement Benefits (Details 4) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in plan assets
Fair value of plan assets at beginning of year	$ 25,592
Fair value of plan assets at end of year	28,564	$ 25,592
Amounts Recognized on the Balance Sheet
Non-current liability	(5,511)	(6,818)
Pension Benefits
Change in projected benefit obligation
Projected benefit obligation at beginning of year	30,409	29,182
Service cost	424	446	$ 484
Interest cost	1,234	1,284	1,224
Plan participants’ contributions	11	11
Actuarial (gain) loss	1,526	1,026
Benefits paid	(1,617)	(1,480)
Other	(20)	(60)
Projected benefit obligation at end of year	31,967	30,409	29,182
Change in plan assets
Fair value of plan assets at beginning of year	24,384	23,950
Net gain on plan assets	3,885	1,867
Employer contributions	596	81
Plan participants’ contributions	11	11
Benefits paid	(1,617)	(1,480)
Other	(33)	(45)
Fair value of plan assets at end of year	27,226	24,384	23,950
Funded status	(4,741)	(6,025)
Amounts Recognized on the Balance Sheet
Non-current assets	82	2
Current liability	(154)	(146)
Non-current liability	(4,669)	(5,881)
Medical and Life Benefits
Change in projected benefit obligation
Projected benefit obligation at beginning of year	2,100	2,181
Service cost	23	30	35
Interest cost	84	94	94
Plan participants’ contributions	24	20
Actuarial (gain) loss	26	(80)
Benefits paid	(144)	(146)
Other	(3)	1
Projected benefit obligation at end of year	2,110	2,100	2,181
Change in plan assets
Fair value of plan assets at beginning of year	1,208	1,153
Net gain on plan assets	208	97
Employer contributions	45	83
Plan participants’ contributions	24	20
Benefits paid	(144)	(146)
Other	(3)	1
Fair value of plan assets at end of year	1,338	1,208	$ 1,153
Funded status	(772)	(892)
Amounts Recognized on the Balance Sheet
Non-current assets	112	87
Current liability	(42)	(42)
Non-current liability	$ (842)	$ (937)