Employee Benefits - Amounts recognized in Accumulated other comprehensive loss that have not yet been recognized (Details 3) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
U.K.
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost
Net loss	$ 1,217	$ 1,237
Prior service cost (income)	19	19
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost	1,236	1,256
U.S.
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost
Net loss	1,701	1,612
Prior service cost (income)	5	6
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost	1,706	1,618
Other
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost
Net loss	459	400
Prior service cost (income)	(7)	(6)
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost	$ 452	$ 394