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Stock Options and Other Equity-Based Incentive Instruments - Restricted Stock Unit (RSU) Activity (Detail) - Restricted Stock Units (RSUs) [Member]
12 Months Ended
Dec. 31, 2017
$ / shares
shares
Market Plus Service Condition Vesting RSUs [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
RSUs outstanding, Number of RSUs, beginning balance 47,426
RSUs granted, Number of RSUs 17,939
Additional market plus service condition RSUs vested 6,269
RSUs released, Number of RSUs (18,805)
RSUs outstanding, Number of RSUs, ending balance 52,829
RSUs granted, Weighted Average Fair Value on Date of Grant | $ / shares $ 32.95
Service Condition Vesting RSUs [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
RSUs outstanding, Number of RSUs, beginning balance 68,450
RSUs granted, Number of RSUs 29,669
RSUs added through dividend credits, Number of RSUs 1,245
RSUs released, Number of RSUs (30,896)
RSUs forfeited/expired, Number of RSUs (11)
RSUs outstanding, Number of RSUs, ending balance 68,457
RSUs granted, Weighted Average Fair Value on Date of Grant | $ / shares $ 35.36