Quarterly report pursuant to Section 13 or 15(d)

Summary of Share Activity Related to Restricted Stock Units (Detail)

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Summary of Share Activity Related to Restricted Stock Units (Detail) (Restricted Stock Units (RSUs), USD $)
6 Months Ended
Jun. 30, 2013
Restricted Stock Units (RSUs)
 
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Shares, Nonvested, beginning of period 969,820
Shares, Granted 964,780
Shares, Forfeited (18,681)
Shares, Nonvested, end of period 1,915,919
Weighted Average Grant Date Value Per Share, Beginning of period $ 22.70
Weighted Average Grant Date Fair Value Per Share, Granted $ 13.22
Weighted Average Grant Date Fair Value Per Share, Forfeited $ 19.36
Weighted Average Grant Date Value Per Share, End of period $ 17.96