Employee Benefits - Amounts recognized in Accumulated other comprehensive loss that have not yet been recognized (Details 3) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
U.K. [Member]
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost
Net loss	$ 1,465	$ 1,687
Prior service cost (income)	46	22
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost	1,511	1,709
U.S. [Member]
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost
Net loss	1,723	1,737
Prior service cost (income)	9	11
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost	1,732	1,748
Other [Member]
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost
Net loss	389	456
Prior service cost (income)	(7)	3
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost	$ 382	$ 459