Significant Accounting Policies - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) Segment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments | Segment	2
Percentage of original cost of depreciable property	3.50%	3.70%	3.70%
Depreciation expense	$ 303.6	$ 306.0	$ 295.8
Allowance for funds used during construction rate	6.46%	6.46%	6.46%
Allowance for funds used during construction	$ 35.6	$ 25.5	$ 15.6
Unbilled revenues	53.6	53.7
Purchased power	104.1	78.9	71.4
Franchise fees and gross receipts taxes	$ 116.9	$ 116.9	$ 113.9
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Discount rates used in estimating other self-insurance liabilities	2.69%	2.92%
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Discount rates used in estimating other self-insurance liabilities	4.00%	4.00%
PGS [Member]
Summary Of Significant Accounting Policies [Line Items]
Reduction in annual depreciation expense	$ 16.1