Employee Benefits - Amounts recognized in Accumulated other comprehensive loss that have not yet been recognized (Details 3) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
U.K.
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost
Net loss	$ 1,237	$ 1,465
Prior service cost (income)	19	46
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost	1,256	1,511
U.S.
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost
Net loss	1,612	1,723
Prior service cost (income)	6	9
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost	1,618	1,732
Other
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost
Net loss	400	389
Prior service cost (income)	(6)	(7)
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost	$ 394	$ 382