Revenue from Contracts With Customers - Performance obligations (Details) - Aerospace $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Sales expected to be recognized on multi-year contracts in place as of the end of the period	$ 1,639
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-04-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Period in which remaining performance obligations expect to be satisfied and revenue recognized	12 months
Sales expected to be recognized on multi-year contracts in place as of the end of the period	$ 862
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-04-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Period in which remaining performance obligations expect to be satisfied and revenue recognized	0 months
Sales expected to be recognized on multi-year contracts in place as of the end of the period	$ 777