Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Apr. 30, 2018	Nov. 30, 2016
Common Class A [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock repurchase program, authorized amount				$ 200,000,000
2016 Share Repurchase Program [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Treasury stock acquired, average cost per share	$ 3.51	$ 3.59	$ 2.78
Stock repurchase program, aggregate number of shares repurchased value	$ 24,200,000	$ 65,400,000	$ 101,000,000
2016 Share Repurchase Program [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock repurchase program, authorized amount					$ 200,000,000
2016 Share Repurchase Program [Member] | Common Class A [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Repurchase of common stock	6.9	18.2	36.3