Restructuring (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2019
Employee Severance and Benefits
Restructuring Cost and Reserve [Line Items]
Restructuring expenses	$ 78	$ 25	$ 40	$ 78