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Differences Between Provisions for Income Taxes and Provisions Calculated at Statutory Federal Tax Rate (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Examination [Line Items]      
Statutory Federal Income Tax Rate 34.00% 34.00% 34.00%
State Income Taxes, net 6.00% 4.00% 5.00%
Utility Plant Differences (1.00%) (1.00%) (2.00%)
Tax Credits (1.00%) (1.00%) (1.00%)
Other, net     1.00%
Effective Income Tax Rate 38.00% 36.00% 37.00%