Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

General descriptive information regarding an arrangement under which employees and directors receive: (a) awards of equity shares, equity share options, or other equity instruments; or (b) the entity incurs liabilities to them: (1) in amounts based, at least in part, on the price of the entity's shares or other equity instruments; or (2) that require or may require settlement by issuance of the entity's shares. Such an arrangement is usually provided to employees and directors to compensate them, provide performance incentives to them, and to attract or retain their services. May also include pertinent information particular to a plan that is not elsewhere specified in the taxonomy.

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Stock-based Compensation - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended					3 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended			3 Months Ended	9 Months Ended
	Sep. 30, 2011 Performance Unit Awards 2009	Sep. 30, 2010 Performance Unit Awards 2009	Sep. 30, 2011 Performance Unit Awards 2009	Sep. 30, 2010 Performance Unit Awards 2009	Sep. 30, 2011 Performance Unit Awards 2009 Minimum	Sep. 30, 2011 Performance Unit Awards 2009 Maximum	Sep. 30, 2011 Performance Unit Awards 2009 Maximum Certain Circumstances	Sep. 30, 2011 Performance Unit Awards 2010	Sep. 30, 2011 Performance Unit Awards 2010	Dec. 31, 2010 Performance Unit Awards 2010	Sep. 30, 2011 Performance Unit Awards 2010 Minimum	Sep. 30, 2011 Performance Unit Awards 2010 Maximum	Sep. 30, 2011 Performance Unit Awards 2010 Maximum Certain Circumstances	Sep. 30, 2011 Performance Unit Awards 2011	Sep. 30, 2011 Performance Unit Awards 2011	Sep. 30, 2011 Performance Unit Awards 2011 Minimum	Sep. 30, 2011 Performance Unit Awards 2011 Maximum	Sep. 30, 2011 Performance Unit Awards 2011 Maximum Certain Circumstances
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Performance period			The performance period for the 2009 Performance Units is the period from April 1, 2009 through March 31, 2012, but can extend through March 31, 2014 in certain circumstances.		April 1, 2009	March 31, 2012	March 31, 2014		The performance period for the 2010 Performance Units is the period from April 1, 2010 through March 31, 2013, but can extend through March 31, 2015 in certain circumstances.		April 1, 2010	March 31, 2013	March 31, 2015		The performance period for the 2011 Performance Units is the period from April 1, 2011 through March 31, 2014, but can extend through March 31, 2016 in certain circumstances.	April 1, 2011	March 31, 2014	March 31, 2016
Award description			Generally, the recipients will receive a base payment if the Company's total shareholder return is positive and, when compared to the peer group, is at or above the 25th percentile but less than the 50th percentile, two times the base if at or above the 50th percentile but less than the 75th percentile, and four times the base if at the 75th percentile or higher.						Generally, the recipients will receive a base number of shares if the Company’s total shareholder return is positive and, when compared to the peer group, is at the 25th percentile, two times the base if at the 50th percentile, and four times the base if at the 75th percentile or higher. The grant of shares when achievement is between the 25th and 75th percentile will be determined on a pro-rata basis.						Generally, the recipients will receive a base number of shares if the Company’s total shareholder return is positive and, when compared to the peer group, is at the 25th percentile, two times the base if at the 50th percentile, and four times the base if at the 75th percentile or higher. The grant of shares when achievement is between the 25th and 75th percentile will be determined on a pro-rata basis.
Base payment			$ 1,700,000
Pro-rated obligation	4,600,000		4,600,000
Compensation expense	0	751,000	2,200,000	808,000				260,000	779,000					464,000	928,000
Base number of shares									89,375						72,188
Grant date fair value										$ 3,100,000					$ 5,600,000