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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, 2016
$ / shares
shares
Nonvested Restricted Share Awards  
Beginning Balance | shares 938,863
Vested | shares (169,567)
Forfeited | shares (38,629)
Change in attainment for 2015 grants | shares (18,232)
Ending Balance | shares 712,435
Weighted-Average Grant Date Fair Value  
Beginning Balance | $ / shares $ 23.42
Vested | $ / shares 24.41
Forfeited | $ / shares 22.32
Change in attainment for 2015 grants | $ / shares 24.41
Ending Balance | $ / shares $ 23.22