Stock Repurchase Plan - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jul. 26, 2016
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased (in shares)		400,500,000	1,184,700,000
2016 Stock Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Stock repurchase plan (up to)				$ 100,000,000
2017 Stock Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased (in shares)	0