Employee Benefits - Amounts recognized in Accumulated other comprehensive loss that have not yet been recognized (Details 3) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
U.K.
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost
Net loss	$ 1,106	$ 1,217
Prior service cost (income)	30	19
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost	1,136	1,236
U.S.
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost
Net loss	1,705	1,701
Prior service cost (income)	3	5
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost	1,708	1,706
Other
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost
Net loss	440	459
Prior service cost (income)	(7)	(7)
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost	$ 433	$ 452