Summary of Calculation of Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Numerator:
Net income	$ 24,226	$ 19,876	$ 43,166	$ 45,362
Less: income allocated to unvested restricted shares	(195)	(159)	(359)	(345)
Net income attributable to common shareholders - basic	24,031	19,717	42,807	45,017
Add: undistributed income attributable to unvested restricted shares - basic	42	19	63	68
Less: undistributed income attributable to unvested restricted shares - diluted	(42)	(19)	(63)	(68)
Net income attributable to common shareholders - diluted	$ 24,031	$ 19,717	$ 42,807	$ 45,017
Denominator:
Weighted average shares outstanding - basic (in shares)	62,226	62,311	62,281	62,271
Dilutive shares - stock options (in shares)	0	5	1	9
Weighted average shares outstanding - diluted (in shares)	62,226	62,316	62,282	62,280
Net income per share attributable to common shareholders:
Basic (in usd per share)	$ 0.39	$ 0.32	$ 0.69	$ 0.72
Diluted (in usd per share)	$ 0.39	$ 0.32	$ 0.69	$ 0.72