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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] - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Number of Shares at Expected Attainment      
Beginning Balance 1,738,056 889,295 1,145,916
Granted 553,549 1,059,428 1,025,863
Vested     (548,671)
Forfeited (201,441) (210,667) (205,510)
Change in expected attainment for 2011 and 2013 grants (965,129)   (528,303)
Ending Balance 1,125,035 1,738,056 889,295
Weighted-Average Grant Date Fair Value      
Beginning Balance $ 18.45 $ 19.13 $ 14.84
Granted 20.12 17.92 19.12
Vested     9.75
Forfeited 18.87 18.61 19.39
Change in expected attainment for 2011 and 2013 grants 18.75   19.44
Ending Balance $ 18.94 $ 18.45 $ 19.13