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Stock-Based Compensation Plans - Summary of Nonvested LTIP Performance Based Share Awards and Changes During Period (Detail) - LTIP Performance Shares [Member]
9 Months Ended
Sep. 30, 2017
$ / shares
shares
Number of shares at expected attainment  
Beginning Balance | shares 1,738,056
Granted | shares 553,549
Forfeited | shares (150,241)
Ending Balance | shares 2,141,364
Weighted-Average Grant Date Fair Value  
Beginning Balance | $ / shares $ 18.45
Granted | $ / shares 20.12
Forfeited | $ / shares 18.77
Ending Balance | $ / shares $ 18.86