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Provisions for Federal and State Income Taxes (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Deferred Federal Tax Provision (Benefit)      
Deferred Federal Tax Provision (Benefit) $ 12,413 $ 10,809 $ 10,052
Total Federal Tax Provision 12,413 10,809 10,052
State      
Current 3,530 (387) 386
Deferred (500) 3,573 2,214
Total State Tax Provision 3,030 3,186 2,600
Total Provision for Federal and State Income Taxes 15,443 13,995 12,652
Operating Loss Carryforwards      
Current Federal Tax Provision (Benefit)      
Current Federal Tax Provision (Benefit) (3,710) (3,179)  
Utility Plant Differences      
Deferred Federal Tax Provision (Benefit)      
Deferred Federal Tax Provision (Benefit) 17,924 10,649 28,907
Net Operating Loss Carryforwards Carrybacks      
Deferred Federal Tax Provision (Benefit)      
Deferred Federal Tax Provision (Benefit) 2,374 2,589 (8,053)
Regulatory Assets and Liabilities      
Deferred Federal Tax Provision (Benefit)      
Deferred Federal Tax Provision (Benefit) (6,101) (5,946) (11,483)
Other, net      
Deferred Federal Tax Provision (Benefit)      
Deferred Federal Tax Provision (Benefit) (1,784) 3,517 $ 681
Operating Income      
Current Federal Tax Provision (Benefit)      
Current Federal Tax Provision (Benefit) $ 3,710 $ 3,179