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Equity and Cash Incentive Program - Narrative (Detail) - USD ($)
1 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2017
May 31, 2018
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Share-based compensation arrangement by share-based payment award, number of shares authorized     6,500,000      
Minimum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Restrictions lapse period       2 years    
Maximum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Restrictions lapse period       3 years    
Stock Appreciation Rights [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period     3 years 3 years    
Expiration period     10 years 10 years    
SARs granted       82,240 106,548 98,803
Pricing model       Black-Scholes    
Unrecognized compensation expense $ 672,000     $ 672,000    
Weighted Average period for Unrecognized compensation expense period       1 year 8 months 12 days    
Performance Share Awards [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
SARs granted   87,000   4,162    
Share Based Compensation Arrangement       3 years    
Stock Compensation Vesting shares       4,162 5,764 4,503
Unvested Performance Based Options [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Unrecognized compensation expense 0     $ 0    
Weighted Average period for Unrecognized compensation expense period       1 year    
Restricted Stock            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
SARs granted   352,000   25,557 42,117 17,263
Unvested Restricted Stock [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Unrecognized compensation expense $ 2,060     $ 2,060    
Weighted Average period for Unrecognized compensation expense period 1 year 7 months 6 days