Summary of Calculation of Net Income Per Common Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Numerator:
Net income	$ 7,155	$ 27,970	$ 52,516	$ 82,941
Less: income allocated to unvested restricted shares	(150)	(186)	(453)	(541)
Net income attributable to common shareholders - basic	7,005	27,784	52,063	82,400
Add: undistributed income attributable to unvested restricted shares - basic	0	59	26	185
Less: undistributed income attributable to unvested restricted shares - diluted	0	(59)	(26)	(185)
Net income attributable to common shareholders - diluted	$ 7,005	$ 27,784	$ 52,063	$ 82,400
Denominator:
Weighted average shares outstanding - basic (in shares)	62,175	62,605	62,238	62,678
Dilutive shares - stock options (in shares)	4	26	7	41
Weighted average shares outstanding - diluted (in shares)	62,179	62,631	62,245	62,719
Net income per share attributable to common shareholders:
Basic (in usd per share)	$ 0.11	$ 0.44	$ 0.84	$ 1.31
Diluted (in usd per share)	$ 0.11	$ 0.44	$ 0.84	$ 1.31