Accounting Policies - Unearned Revenue (Details) - USD ($) $ in Billions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Unearned revenue		$ 10.2
Unearned revenue, revenue recognized from beginning balance	$ 3.4
Unearned Revenue, Noncurrent	1.8	$ 2.0
Remaining performance obligation, contracts exceeding one year	$ 34.1
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-04-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction, weighted average remaining life	3 years 3 months 18 days