Supplemental Stockholders' Equity Information - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Jan. 14, 2015
Class of Stock [Line Items]
Stock repurchase program, remaining authorized repurchase amount	$ 16,200,000
Class A Common Stock
Class of Stock [Line Items]
Stock repurchase program, authorized amount		$ 101,100,000
Shares repurchased (in shares)	6,690,905
Share repurchases	$ 84,900,000