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Benefit Plans - Status of Unvested Stock Awards (Detail) - shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Restricted stock awards, outstanding at beginning of year 547,698 591,746 846,462
Granted 288,519 351,836 339,936
Forfeited (62,677) (178,632) (240,191)
Vested (112,757) (217,252) (354,461)
Restricted stock awards, outstanding at the end of year 660,783 547,698 591,746