Summary of Calculation of Net Income Per Common Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Numerator:
Net income	$ 19,876	$ 28,872	$ 45,362	$ 54,970
Less: income allocated to unvested restricted shares	(159)	(156)	(345)	(351)
Net income attributable to common shareholders - basic	19,717	28,716	45,017	54,619
Add: undistributed income attributable to unvested restricted shares - basic	19	64	68	122
Less: undistributed income attributable to unvested restricted shares - diluted	(19)	(64)	(68)	(122)
Net income attributable to common shareholders - diluted	$ 19,717	$ 28,716	$ 45,017	$ 54,619
Denominator:
Weighted average shares outstanding - basic (in shares)	62,311	62,707	62,271	62,695
Dilutive shares - stock options (in shares)	5	45	9	51
Weighted average shares outstanding - diluted (in shares)	62,316	62,752	62,280	62,746
Net income per share attributable to common shareholders:
Basic (in usd per share)	$ 0.32	$ 0.46	$ 0.72	$ 0.87
Diluted (in usd per share)	$ 0.32	$ 0.46	$ 0.72	$ 0.87