Accounting Policies - Unearned Revenue (Details) - USD ($) $ in Billions	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Unearned revenue		$ 7.9
Unearned revenue, revenue recognized from beginning balance	$ 4.6
Unearned Revenue, Noncurrent	1.6	$ 1.4
Remaining performance obligation, contracts exceeding one year	$ 25.1
Remaining performance obligation, expected timing of satisfaction, weighted average remaining life	3 years 4 months 24 days