Computation of Basic and Diluted Earnings per Share (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Numerator:
Net income	$ 8,622,613	$ 5,994,840	$ 6,093,744	$ 5,627,551	$ 5,523,037	$ 6,831,221	$ 5,533,229	$ 5,054,995	$ 26,338,748	$ 22,942,482	$ 18,363,047
Less: Income allocated to participating securities									(299,518)	(294,022)	(556,304)
Net income available to common stockholders									$ 26,039,230	$ 22,648,460	$ 17,806,743
Denominator:
Weighted average shares - basic	64,627,150	64,585,659	64,488,837	64,296,996	64,176,446	64,120,253	63,997,854	63,771,121	64,500,244	64,017,813	63,511,383
Performance-based restricted stock awards									64,420	138,856	97,697
Weighted average shares - diluted	64,696,246	64,603,817	64,488,837	64,296,996	64,324,735	64,199,973	64,060,660	63,831,274	64,564,664	64,156,669	63,609,080
Basic net income per share:
Net income	$ 0.13	$ 0.09	$ 0.09	$ 0.09	$ 0.08	$ 0.11	$ 0.09	$ 0.08	$ 0.40	$ 0.35	$ 0.28
Diluted net income per share:
Net income	$ 0.13	$ 0.09	$ 0.09	$ 0.09	$ 0.08	$ 0.11	$ 0.09	$ 0.08	$ 0.40	$ 0.35	$ 0.28