Treasury Stock (Detail) - USD ($)	3 Months Ended	9 Months Ended		117 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2013
Equity [Abstract]
Common stock amount authorized under repurchase program, value (up to)					$ 189,800,000.0	$ 150,000,000.0
Additional authorized amount					$ 39,800,000.0
Remaining authorized repurchase amount	$ 30,500,000	$ 30,500,000		$ 30,500,000
Repurchase of common stock (in shares)	349,737	2,175,268	1,010,000	13,761,211
Payment for repurchase of common stock, net of fees		$ 30,400,000	$ 9,500,000
Average stock price of shares repurchased (in dollars per share)		$ 13.95	$ 9.41	$ 11.58
Payments for treasury stock		$ 30,434,000	$ 9,533,000	$ 159,300,000