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Stock-Based Compensation Plans - Summary of Nonvested Performance Based Share Awards and Changes During Period (Detail) - Performance-Based Restricted Share Awards [Member]
6 Months Ended
Jun. 30, 2017
$ / shares
shares
Nonvested Restricted Share Awards  
Beginning Balance | shares 683,667
Vested | shares (484,835)
Forfeited | shares (11,604)
Change in attainment for 2015 grants | shares (13,592)
Ending Balance | shares 173,636
Weighted-Average Grant Date Fair Value  
Beginning Balance | $ / shares $ 23.25
Vested | $ / shares 22.82
Forfeited | $ / shares 23.84
Change in attainment for 2015 grants | $ / shares 23.25
Ending Balance | $ / shares $ 24.41