Summary of Calculation of Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Numerator:
Net income	$ 18,785	$ 24,135
Less: income allocated to unvested restricted shares	(239)	(276)
Net income attributable to common shareholders - basic	18,546	23,859
Add: undistributed income attributable to unvested restricted shares - basic	23	57
Less: undistributed income attributable to unvested restricted shares - diluted	(23)	(57)
Net income attributable to common shareholders - diluted	$ 18,546	$ 23,859
Denominator:
Weighted average shares outstanding - basic and diluted (in shares)	60,013	61,696
Net income per share attributable to common shareholders:
Basic (in usd per share)	$ 0.31	$ 0.39
Diluted (in usd per share)	$ 0.31	$ 0.39