SUMMARY OF QUARTERLY RESULTS (UNAUDITED) (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 95,012	$ 87,108	$ 83,825	$ 84,515	$ 76,304	$ 72,818	$ 72,606	$ 70,476	$ 350,460	$ 292,204	$ 258,321
Net (loss) income	33,923	30,119	30,572	26,084	19,293	21,436	17,790	(5,913)	120,698	52,606	58,436
Net (loss) income available to common stockholders	$ 33,923	$ 30,119	$ 30,572	$ 26,084	$ 19,293	$ 21,436	$ 17,806	$ (11,076)	$ 120,698	$ 47,459	$ 49,350
Net income available to common per share:
Basic net (loss) income	$ 0.30	$ 0.28	$ 0.29	$ 0.25	$ 0.19	$ 0.21	$ 0.17	$ (0.11)	$ 1.12	$ 0.46	$ 0.52
Diluted net (loss) income	$ 0.30	$ 0.27	$ 0.29	$ 0.25	$ 0.19	$ 0.21	$ 0.17	$ (0.11)	$ 1.12	$ 0.46	$ 0.52
Cash dividends paid on common stock	$ 0.44	$ 0.42	$ 0.42	$ 0.41	$ 0.40	$ 0.40	$ 0.38	$ 0.37	$ 1.69	$ 1.55	$ 1.37