Business Combinations - Fair Value Adjustments (Details) $ in Thousands	Apr. 13, 2016 USD ($)
Property, plant, and equipment
Business Acquisition [Line Items]
Assets, fair value adjustments	$ (1,334,932)
Accumulated depreciation
Business Acquisition [Line Items]
Assets, fair value adjustments	(1,565,776)
Goodwill
Business Acquisition [Line Items]
Assets, fair value adjustments	1,490,797
Intangible assets
Business Acquisition [Line Items]
Assets, fair value adjustments	91,826
Regulatory assets
Business Acquisition [Line Items]
Assets, fair value adjustments	250,409
Deferred income tax liabilities
Business Acquisition [Line Items]
Liabilities, fair value adjustments	126,853
Other deferred credits
Business Acquisition [Line Items]
Liabilities, fair value adjustments	21,175
Long-term debt
Business Acquisition [Line Items]
Liabilities, fair value adjustments	$ 198,599