Description of Business and Accounting Policies - Unearned Revenue (Details) - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Contract with Customer, Liability		$ 6.1
Contract with Customer, Liability, Revenue Recognized	$ 5.3
Contract with Customer, Liability, Noncurrent	1.4	$ 1.0
Revenue, Remaining Performance Obligation, Amount	$ 19.3
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	3 years 4 months