Employee Benefits - Reconciliation of the changes in the benefit obligations and fair value of assets and a statement of the funded status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
U.K. [Member]
Change in projected benefit obligation
Balance at the beginning of the Period	$ 5,529	$ 5,106
Service cost	1	1	$ 1
Interest cost	198	230	210
Participant contributions	0	0
Plan amendment	27	0
Curtailments	0	0
Plan transfer and acquisitions	(2)	0
Actuarial loss (gain)	(83)	(211)
Benefit payments	(217)	(192)
Actual Expenses	0	0
Change in discount rate	(247)	902
Foreign currency impact	(221)	(307)
Balance at the end of the period	4,985	5,529	5,106
Accumulated benefit obligation at end of year	4,985	5,529
Change in fair value of plan assets
Balance at the beginning of the period	6,224	5,398
Actual return on plan assets	91	1,199
Participant contributions	0	0
Employer contributions	65	166
Plan transfer and acquisitions	(3)	0
Benefit payments	(217)	(192)
Actual Expenses	0	0
Foreign currency impact	(257)	(347)
Balance at the end of the period	5,903	6,224	5,398
Market related value at end of year	5,903	6,224
Funded status	918	695
Unrecognized prior-service cost	46	22
Unrecognized loss	1,465	1,687
Net amount recognized	2,429	2,404
U.S. [Member]
Change in projected benefit obligation
Balance at the beginning of the Period	3,350	2,744
Service cost	0	2	7
Interest cost	131	129	114
Participant contributions	0	0
Plan amendment	0	0
Curtailments	0	0
Plan transfer and acquisitions	(18)	13
Actuarial loss (gain)	(25)	265
Benefit payments	(133)	(130)
Actual Expenses	0	0
Change in discount rate	(145)	327
Foreign currency impact	0	0
Balance at the end of the period	3,160	3,350	2,744
Accumulated benefit obligation at end of year	3,160	3,350
Change in fair value of plan assets
Balance at the beginning of the period	2,036	1,855
Actual return on plan assets	(60)	190
Participant contributions	0	0
Employer contributions	108	121
Plan transfer and acquisitions	0	0
Benefit payments	(133)	(130)
Actual Expenses	0	0
Foreign currency impact	0	0
Balance at the end of the period	1,951	2,036	1,855
Market related value at end of year	2,064	1,950
Funded status	(1,209)	(1,314)
Unrecognized prior-service cost	9	11
Unrecognized loss	1,723	1,737
Net amount recognized	523	434
Other [Member]
Change in projected benefit obligation
Balance at the beginning of the Period	1,399	1,252
Service cost	0	0	18
Interest cost	33	47	45
Participant contributions	0	0
Plan amendment	(10)	0
Curtailments	0	(16)
Plan transfer and acquisitions	0	0
Actuarial loss (gain)	24	(5)
Benefit payments	(38)	(51)
Actual Expenses	0	(2)
Change in discount rate	(66)	324
Foreign currency impact	(165)	(150)
Balance at the end of the period	1,177	1,399	1,252
Accumulated benefit obligation at end of year	1,135	1,316
Change in fair value of plan assets
Balance at the beginning of the period	1,161	1,061
Actual return on plan assets	8	253
Participant contributions	0	0
Employer contributions	21	28
Plan transfer and acquisitions	0	0
Benefit payments	(38)	(51)
Actual Expenses	0	(2)
Foreign currency impact	(133)	(128)
Balance at the end of the period	1,019	1,161	$ 1,061
Market related value at end of year	1,019	1,161
Funded status	(158)	(238)
Unrecognized prior-service cost	(7)	3
Unrecognized loss	389	456
Net amount recognized	$ 224	$ 221