| http://www.w3.org/ns/prov#value | - The risk-free interest rate is based on the U.S. Treasury yield curve in effect at the time of the grant.Performance Share UnitsDuring the three months ended March 31, 2013, the Company issued to certain key employees a total of 217,730 performance share units, which vest on the third anniversary of the grant date, subject to the achievement of certain performance goals.
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