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Deferred Tax Assets and Liabilities Current, Non Current (Detail) - USD ($) $ in Thousands | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | ---
Schedule of Deferred Tax Assets and Liabilities [Line Items] |  |
Utility Plant Differences | $ 141,185 | $ 120,534
Retirement Benefit Obligations | (43,543) | (44,829)
Net Operating Loss Carryforwards | (10,500) | (13,122)
Regulatory Assets & Liabilities | 10,535 | 12,740
AMT Tax Credit Carryforwards | (2,677) | (2,139)
Other, net | (2,792) | (314)
Total Noncurrent Deferred Income Tax Liabilities | $ 92,208 | $ 72,870

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