Description of award terms as to how many shares or portion of an award are no longer contingent on satisfaction of either a service condition, market condition or a performance condition, thereby giving the employee the legal right to convert the award to shares, to sell the shares, and be entitled to the cash proceeds of such sale. For example, vesting may be expressed as being 25 percent of the shares under option on each anniversary of the grant date.

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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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Share-Based Compensation - Additional Information (Detail)	1 Months Ended			3 Months Ended
	Feb. 29, 2012 Performance Based Restricted Units	Feb. 29, 2012 Performance Based Restricted Units Minimum	Feb. 29, 2012 Performance Based Restricted Units Maximum	Mar. 31, 2012 Stock Options	Mar. 31, 2012 Deferred Common Stock Units	Mar. 31, 2012 Restricted Stock and Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				3 years		3 years
Vesting description				One-third vesting in each year	Immediately	Either at the end of the three years or with one-third vesting in each year
Performance based restricted stock units description	During February 2012, we granted performance-based restricted stock units (“PBRSUs”) to certain executive employees. The PBRSUs are payable in cash on the third anniversary of the date of grant based upon the achievement of certain market-based performance metrics with respect to each of a one-year, two-year and three-year performance period, in each case commencing on the date of grant.
PBRSUs vesting percentage		0.00%	200.00%