Stockholders' Equity - Changes in Common Shares and Treasury Stock (Details) - USD ($)		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Common stock, shares authorized		6,000,000,000		6,000,000,000
Preferred stock, shares authorized		1,000,000,000		1,000,000,000
Stock repurchase program, remaining authorized repurchase amount		$ 151,000,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issued, shares		501,891,243	501,890,000
Treasury stock, shares	[1]	22,328,917	15,760,000
Stock issued, employee stock purchase plans, shares		0	0
Stock issued, employee benefit plan, shares		0	0
Common stock issued, shares		501,891,243	501,890,000
Treasury stock, shares	[1]	22,865,242	17,166,998
Stock Compensation Plan
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Treasury stock acquired, shares	[1]	(1,140,000)	(990,000)
Share Repurchase Program
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Treasury stock acquired, shares	[1]	(1,670,000)	(2,400,000)
December 2016 Share Repurchase Program
Class of Stock [Line Items]
Stock repurchase program, authorized amount					$ 1,500,000,000
December 2018 Share Repurchase Program [Member]
Class of Stock [Line Items]
Stock repurchase program, authorized amount				$ 2,000,000,000.0

[1] Shares may not add due to rounding.