Financial Instruments & Related Fair Value (Details) MMBTU in Millions	2 Months Ended
	Dec. 31, 2016 USD ($) MMBTU $ / MMBTU	Dec. 31, 2015 USD ($)	Oct. 31, 2016 USD ($) MMBTU
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gas Options Total Coverage | MMBTU	15.7		15.4
Derivative Asset, Collateral, Obligation to Return Cash, Offset	$ 0		$ 0
Fair Value Measurement Transfers Between Levels Activity	0	$ 0
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	0		0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Current Liabilities - Gas supply derivative	$ 34,400,000		41,500,000
Percentage Of Annual Gas Purchase Options and All Other Costs Related To Hedging Approved For Recovery Under TIP	1.00%
Long-term debt, principal	$ 1,835,000,000		1,835,000,000
Concentration Risk [Line Items]
Concentration Risk, Benchmark Description	"Receivables" within "Current Assets" on the Condensed Consolidated Balance Sheets
Fair Value, Measurements, Recurring
Assets [Abstract]
Total Recurring Fair Value Assets	$ 8,100,000		5,700,000
Money Market Funds | Fair Value, Measurements, Recurring
Assets [Abstract]
Marketable Securities	600,000		500,000
Equity Funds | Fair Value, Measurements, Recurring
Assets [Abstract]
Marketable Securities	4,100,000		3,700,000
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Assets [Abstract]
Total Recurring Fair Value Assets	8,100,000		5,700,000
Fair Value, Inputs, Level 1 | Money Market Funds | Fair Value, Measurements, Recurring
Assets [Abstract]
Marketable Securities	600,000		500,000
Fair Value, Inputs, Level 1 | Equity Funds | Fair Value, Measurements, Recurring
Assets [Abstract]
Marketable Securities	4,100,000		3,700,000
Fair Value, Inputs, Level 2
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Long-term debt, fair value	1,932,600,000		2,061,200,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Assets [Abstract]
Total Recurring Fair Value Assets	0		0
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Assets [Abstract]
Total Recurring Fair Value Assets	0		0
Price Risk Derivative
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	0		0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Current Assets - Gas purchase derivative assets	3,400,000		1,500,000
Amount of Gain (Loss) Recognized on Derivative Instruments	200,000	(1,200,000)
Amount Of Gain (Loss) Deferred Under PGA Procedures	200,000	(1,200,000)
Price Risk Derivative | Fair Value, Measurements, Recurring
Assets [Abstract]
Derivative Asset	3,400,000		1,500,000
Price Risk Derivative | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Assets [Abstract]
Derivative Asset	3,400,000		1,500,000
Energy Related Derivative
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Current Liabilities - Gas supply derivative	34,400,000		41,500,000
Noncurrent Liabilities - Gas supply derivative liabilities	152,300,000		146,400,000
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss)	1,200,000	(149,400,000)
Energy Related Derivative | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	186,700,000		187,900,000
Energy Related Derivative | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	0		0
Energy Related Derivative | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	0		0
Energy Related Derivative | Fair Value, Inputs, Level 3
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Derivative Liability, beginning balance	187,900,000	0
Recorded to regulatory assets	(1,200,000)	149,400,000
Purchases, sales and settlements (net)	0	0
Transfer in/out of Level 3	0	0
Derivative Liability, ending balance	186,700,000	$ 149,400,000
Energy Related Derivative | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	186,700,000		$ 187,900,000
Credit Concentration Risk
Concentration Risk [Line Items]
Accounts Receivable, Net, Current	$ 16,200,000
Minimum | Energy Related Derivative
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Price Per Dekatherm | $ / MMBTU	2.31
Maximum | Energy Related Derivative
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Price Per Dekatherm | $ / MMBTU	4.18
Accounts Receivable | Credit Concentration Risk
Concentration Risk [Line Items]
Concentration Risk, Percentage	11.00%