Employee Benefit Plans (Details) £ in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($)	Mar. 31, 2017 GBP (£)	Mar. 31, 2016 USD ($)
Domestic Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 3
Service cost	6		$ 8
Interest cost	29		31
Expected return on plan assets	(40)		(40)
Net amortization	7		11
Net periodic benefit cost	2		10
Domestic Other Postretirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	4
Service cost	2		3
Interest cost	6		8
Expected return on plan assets	(10)		(11)
Net amortization	(3)		(3)
Net periodic benefit cost	(5)		(3)
UK Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	12	£ 10
Service cost	6		5
Interest cost	14		19
Expected return on plan assets	(24)		(29)
Net amortization	17		12
Net periodic benefit cost	$ 13		$ 7