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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