Employee Benefits - Amounts recognized in the Consolidated Statements of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
U.K.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prepaid benefit cost	$ 836	$ 1,012
Accrued benefit liability	(35)	(94)
Accumulated other comprehensive loss	1,256	1,511
Net amount recognized	2,057	2,429
U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prepaid benefit cost	0	0
Accrued benefit liability	(1,225)	(1,209)
Accumulated other comprehensive loss	1,618	1,732
Net amount recognized	393	523
Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prepaid benefit cost	0	0
Accrued benefit liability	(151)	(158)
Accumulated other comprehensive loss	394	382
Net amount recognized	$ 243	$ 224