Stock Option Activity (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Total intrinsic value of stock options exercised	$ 45	$ 1	$ 972
Total fair value of share awards vested	$ 25,220	$ 26,892	$ 21,673