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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, 2015
Dec. 31, 2014
Dec. 31, 2013
Number of Shares at Expected Attainment      
Beginning Balance 1,145,916 2,718,576 3,304,452
Granted 1,025,863 19,065 798,306
Vested (548,671) (635,643) (982,728)
Forfeited (205,510) (111,599) (188,511)
Change in expected attainment for 2010 grants (528,303) (844,483) (212,943)
Ending Balance 889,295 1,145,916 2,718,576
Weighted-Average Grant Date Fair Value      
Beginning Balance $ 14.84 $ 13.78 $ 9.38
Granted 19.12 20.13 20.30
Vested 9.75 8.88 5.61
Forfeited 19.39 16.43 12.33
Change in expected attainment for 2010 grants 19.44 15.86 8.88
Ending Balance $ 19.13 $ 14.84 $ 13.78