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Summary of Employee Stock Purchase Plan Activity (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares purchased by employees	248,405	196,332	215,054
Aggregate cost to employees	$ 13,116	$ 12,183	$ 11,273
Expense recognized by the company	59,381	38,601	37,047
Company expense on ESPP discount
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expense recognized by the company	$ 2,315	$ 2,150	$ 1,989

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.

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- Definition

Number of shares issued during the period as a result of an employee stock purchase plan.

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