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Benefit Plans - Assets Measured at Fair Value on Recurring Basis (Detail) - Pension [Member] - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]        
Total investments $ 46,870 $ 43,153 $ 41,448 $ 42,744
Group Annuity Contracts [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 120 127    
Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 21,216 19,535    
Pooled Separate Accounts [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments   23,491    
Level 1 [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 21,216 19,535    
Level 1 [Member] | Group Annuity Contracts [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 0 0    
Level 1 [Member] | Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 21,216 19,535    
Level 1 [Member] | Pooled Separate Accounts [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments   0    
Level 2 [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 25,654 23,618    
Level 2 [Member] | Group Annuity Contracts [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 120 127    
Level 2 [Member] | Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 0 0    
Level 2 [Member] | Pooled Separate Accounts [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments   23,491    
Level 3 [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 0 0    
Level 3 [Member] | Group Annuity Contracts [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 0 0    
Level 3 [Member] | Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 0 0    
Level 3 [Member] | Pooled Separate Accounts [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments   0    
Fixed Income [Member] | Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 13,725 12,740    
Fixed Income [Member] | Pooled Separate Accounts [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 25,534      
Fixed Income [Member] | Level 1 [Member] | Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 13,725 12,740    
Fixed Income [Member] | Level 1 [Member] | Pooled Separate Accounts [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 0      
Fixed Income [Member] | Level 2 [Member] | Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 0 0    
Fixed Income [Member] | Level 2 [Member] | Pooled Separate Accounts [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 25,534      
Fixed Income [Member] | Level 3 [Member] | Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 0 0    
Fixed Income [Member] | Level 3 [Member] | Pooled Separate Accounts [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 0      
International Equity [Member] | Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 5,110 4,659    
International Equity [Member] | Level 1 [Member] | Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 5,110 4,659    
International Equity [Member] | Level 2 [Member] | Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 0 0    
International Equity [Member] | Level 3 [Member] | Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 0 0    
Large U S Equity [Member] | Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 1,431 1,296    
Large U S Equity [Member] | Level 1 [Member] | Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 1,431 1,296    
Large U S Equity [Member] | Level 2 [Member] | Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 0 0    
Large U S Equity [Member] | Level 3 [Member] | Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 0 0    
Small Mid U S Equity [Member] | Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 950 840    
Small Mid U S Equity [Member] | Level 1 [Member] | Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 950 840    
Small Mid U S Equity [Member] | Level 2 [Member] | Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 0 0    
Small Mid U S Equity [Member] | Level 3 [Member] | Mutual Funds [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments $ 0 $ 0