Annual report pursuant to Section 13 and 15(d)

Share-based Compensation - Summary of Transactions for Non-Vested Stock Options (Detail)

v3.8.0.1
Share-based Compensation - Summary of Transactions for Non-Vested Stock Options (Detail) - $ / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward]      
Number of Shares, Non-vested options outstanding at beginning of period 46,340 176,688 379,106
Number of Shares, Options granted 0 0 28,000
Number of Shares, Options vested (37,005) (130,223) (158,810)
Number of Shares, Options forfeited 0 (125) (71,608)
Number of Shares, Non-vested options outstanding at end of period 9,335 46,340 176,688
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]      
Weighted-Average Exercise Price Per Share, Non-vested options outstanding at beginning of period (in usd per share) $ 22.42 $ 19.13 $ 18.11
Weighted-Average Exercise Price Per Share, Options granted (in usd per share) 0.00 0.00 23.47
Weighted-Average Exercise Price Per Share, Options vested (in usd per share) 22.16 17.83 16.80
Weighted-Average Exercise Price Per Share, Options forfeited (in usd per share) 0.00 144.00 20.59
Weighted-Average Exercise Price Per Share, Non-vested options outstanding at end of period (in usd per share) $ 23.47 $ 22.42 $ 19.13