Employee Benefits - Reconciliation of the changes in the benefit obligations and fair value of assets and a statement of the funded status (Details) £ in Millions, $ in Millions	3 Months Ended			12 Months Ended
	Dec. 31, 2016 USD ($)	Jun. 30, 2016 GBP (£)	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
U.K.
Change in projected benefit obligation
Balance at the beginning of the Period				$ 4,985	$ 5,529
Service cost				0	1	$ 1
Interest cost				158	198	230
Plan amendment				(20)	27
Settlements		£ (42)	$ (61)	(159)	0
Plan transfer and acquisitions				0	(2)
Actuarial loss (gain)				32	(83)
Benefit payments		(116)	(159)	(242)	(217)
Change in discount rate				1,079	(247)
Foreign currency impact				(959)	(221)
Balance at the end of the period	$ 4,874			4,874	4,985	5,529
Accumulated benefit obligation at end of year	4,874			4,874	4,985
Change in fair value of plan assets
Balance at the beginning of the period				5,903	6,224
Actual return on plan assets				1,233	91
Employer contributions				67	65
Settlements				(159)	0
Plan transfer and acquisitions				0	(3)
Benefit payments		£ (116)	$ (159)	(242)	(217)
Foreign currency impact				(1,127)	(257)
Balance at the end of the period	5,675			5,675	5,903	6,224
Market related value at end of year	5,675			5,675	5,903
Funded status	801			801	918
Unrecognized prior-service cost	19			19	46
Unrecognized loss	1,237			1,237	1,465
Net amount recognized	2,057			2,057	2,429
U.S.
Change in projected benefit obligation
Balance at the beginning of the Period				3,160	3,350
Service cost				0	0	2
Interest cost				111	131	129
Plan amendment				0	0
Settlements	(158)			(281)	0
Plan transfer and acquisitions				0	(18)
Actuarial loss (gain)				(43)	(25)
Benefit payments	(281)			(139)	(133)
Change in discount rate				100	(145)
Foreign currency impact				0	0
Balance at the end of the period	2,908			2,908	3,160	3,350
Accumulated benefit obligation at end of year	2,908			2,908	3,160
Change in fair value of plan assets
Balance at the beginning of the period				1,951	2,036
Actual return on plan assets				116	(60)
Employer contributions				36	108
Settlements				(281)	0
Plan transfer and acquisitions				0	0
Benefit payments	(281)			(139)	(133)
Foreign currency impact				0	0
Balance at the end of the period	1,683			1,683	1,951	2,036
Market related value at end of year	1,819			1,819	2,064
Funded status	(1,225)			(1,225)	(1,209)
Unrecognized prior-service cost	6			6	9
Unrecognized loss	1,612			1,612	1,723
Net amount recognized	393			393	523
Other
Change in projected benefit obligation
Balance at the beginning of the Period				1,177	1,399
Service cost				0	0	0
Interest cost				29	33	47
Plan amendment				0	(10)
Settlements				0	0
Plan transfer and acquisitions				0	0
Actuarial loss (gain)				(7)	24
Benefit payments				(39)	(38)
Change in discount rate				100	(66)
Foreign currency impact				(33)	(165)
Balance at the end of the period	1,227			1,227	1,177	1,399
Accumulated benefit obligation at end of year	1,191			1,191	1,135
Change in fair value of plan assets
Balance at the beginning of the period				1,019	1,161
Actual return on plan assets				111	8
Employer contributions				20	21
Settlements				0	0
Plan transfer and acquisitions				0	0
Benefit payments				(39)	(38)
Foreign currency impact				(35)	(133)
Balance at the end of the period	1,076			1,076	1,019	$ 1,161
Market related value at end of year	1,076			1,076	1,019
Funded status	(151)			(151)	(158)
Unrecognized prior-service cost	(6)			(6)	(7)
Unrecognized loss	400			400	389
Net amount recognized	$ 243			$ 243	$ 224