Earnings Per Share (Detail) (USD $) In Thousands, except 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
Numerator (both basic and diluted):
Net income (loss) available to common stockholders	$ (19,977)		$ 6,463		$ 14,737		$ 19,631		$ 22,511		$ 26,595		$ 16,550		$ (18,493)		$ 20,854		$ 47,163		$ (43,563)
Denominator:
Denominator for basic earnings per share	39,893,000		40,227,000		41,154,000		40,758,000		40,658,000		40,451,000		40,356,000		39,884,000		40,505,555		40,375,013		39,714,053
Stock options																	167,986		382,436
Unvested restricted stock																	601,752		779,180
Denominator for diluted earnings per share	40,127,000		40,441,000		41,426,000		41,629,000		41,307,000		41,396,000		41,336,000		39,884,000		41,275,293		41,536,629		39,684,231
Basic earnings per common share: Net income (loss) available to common stockholders	$ (0.49)	[1]	$ 0.16	[1]	$ 0.36	[1]	$ 0.48	[1]	$ 0.55	[1]	$ 0.66	[1]	$ 0.41	[1]	$ (0.46)	[1]	$ 0.51	[1]	$ 1.17	[1]	$ (1.10)
Diluted earnings per common share: Net income (loss) available to common stockholders	$ (0.49)	[1]	$ 0.16	[1]	$ 0.36	[1]	$ 0.47	[1]	$ 0.54	[1]	$ 0.64	[1]	$ 0.40	[1]	$ (0.46)	[1]	$ 0.51	[1]	$ 1.14	[1]	$ (1.10)

[1] The sum of individual quarterly net income per share may not agree to the total for the year due to each period's computation being based on the weighted average number of common shares outstanding during each period.