Stockholders' Equity - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	May 08, 2017	Dec. 31, 2014
Stockholders Equity [Line Items]
Common stock, shares issued	92,637,403	91,861,054
Unvested shares of common stock	4,240,991
Treasury stock, shares	6,519,773	6,519,773
Dividends payable	$ 470,000	$ 908,000
Dividends paid to stockholders	1,544,000	65,306,000	$ 72,318,000
Share Repurchase Program, authorized amount					$ 250,000,000
Shares repurchased			2,413,803
Stock repurchased during period, total cost			$ 45,000,000
Treasury stock at cost	154,871,000	$ 154,871,000	$ 154,871,000
Stock Purchase Agreement [Member] | ZHG Group [Member]
Stockholders Equity [Line Items]
Percentage of common stock outstanding				21.00%
Stock Purchase Agreement [Member] | ZHG Group [Member] | Upon Closing of Sale [Member]
Stockholders Equity [Line Items]
Percentage of common stock outstanding				21.00%
Acquisition price paid per share				$ 23.00
Share Repurchase Program [Member]
Stockholders Equity [Line Items]
Shares repurchased			2,413,803
Stock repurchased during period under Share Repurchase Program, average price per share			$ 18.62
Stock repurchased during period, total cost			$ 45,000,000
Share Repurchase Program, remaining authorized repurchase amount	$ 190,000,000
Stock Repurchase Program, number of shares repurchased	0	0
2.75x Modified Performance Restricted Shares [Member]
Stockholders Equity [Line Items]
Dividends paid to stockholders	$ 1,300,000