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Stock-Based Compensation Plans - Summary of Nonvested Performance Based Share Awards and Changes During Period (Detail)
9 Months Ended
Sep. 30, 2015
$ / shares
shares
LTIP Performance Shares [Member]  
Number of Shares at Expected Attainment  
Beginning Balance | shares 1,145,916
Granted | shares 1,025,863
Forfeited | shares (195,919)
Vested | shares (548,671)
Change in expected attainment for 2011 and 2013 grants | shares (528,303)
Ending Balance | shares 898,886
Weighted-Average Grant Date Fair Value  
Beginning Balance $ 14.84
Granted 19.12
Forfeited 19.41
Vested 9.75
Change in expected attainment for 2011 and 2013 grants 19.44
Ending Balance $ 19.13
Performance-Based Restricted Share Awards [Member]  
Number of Shares at Expected Attainment  
Granted | shares 978,365
Forfeited | shares (32,520)
Ending Balance | shares 945,845
Weighted-Average Grant Date Fair Value  
Granted $ 23.45
Forfeited 24.41
Ending Balance $ 23.42