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Stock-Based Compensation Plans - Summary of Nonvested LTIP Performance Shares (Detail) - LTIP Performance Shares [Member]
12 Months Ended
Dec. 31, 2018
$ / shares
shares
Number of Shares at Expected Attainment  
Beginning Balance | shares 1,125,035
Forfeited | shares (93,147)
Change in expected attainment for 2016 grants | shares (491,191)
Ending Balance | shares 540,697
Weighted-Average Grant Date Fair Value  
Beginning Balance | $ / shares $ 18.94
Forfeited | $ / shares 19.26
Change in expected attainment for 2016 grants | $ / shares 17.91
Ending Balance | $ / shares $ 19.83