Fair Value Measurements Assets and Liabilities (Details) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Mar. 31, 2018	Dec. 31, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Excluding Current Maturities		$ 5,658,155		$ 5,658,155			$ 5,257,929
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		5,958	$ 2,099	3,021	$ 1,597
Included in earnings	[1]	67	2,458	2,052	3,503
Included in regulatory assets/liabilities		930	2,133	6,260	5,715
Settlements		(3,162)	(3,084)	(7,418)	(10,227)
Transferred into Level 3		(150)	(1,668)	(1,987)	(30)
Transferred out of Level 3		910	1,726	2,625	3,106
Balance at end of period		4,553	3,664	4,553	3,664
Electric Portfolio [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts		(100)	900	1,400	1,900
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		2,009	643	1,098	972
Included in earnings	[1]	67	2,458	2,052	3,503
Included in regulatory assets/liabilities		0	0	0	0
Settlements		(945)	(1,783)	(1,599)	(5,622)
Transferred into Level 3		(150)	(1,668)	(1,987)	(523)
Transferred out of Level 3		396	1,760	1,813	2,034
Balance at end of period		1,377	1,410	1,377	1,410
Natural Gas Portfolio [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		3,949	1,456	1,923	625
Included in earnings	[1]	0	0	0	0
Included in regulatory assets/liabilities		930	2,133	6,260	5,715
Settlements		(2,217)	(1,301)	(5,819)	(4,605)
Transferred into Level 3		0	0	0	(553)
Transferred out of Level 3		514	(34)	812	1,072
Balance at end of period		3,176	$ 2,254	3,176	$ 2,254
Subsidiaries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Excluding Current Maturities		3,894,487		3,894,487			3,549,911
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets		18,421		18,421			24,405
Derivative Liability		42,146		42,146			86,094
Fair Value, Measurements, Recurring [Member] | Electric Portfolio [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets		8,229		8,229			13,391
Derivative Liability		21,935		21,935			49,050
Fair Value, Measurements, Recurring [Member] | Natural Gas Portfolio [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets		10,192		10,192			11,014
Derivative Liability		20,211		20,211			37,044
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets		11,627		11,627			16,839
Derivative Liability		39,905		39,905			81,549
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Electric Portfolio [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets		5,801		5,801			9,866
Derivative Liability		20,884		20,884			46,623
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Natural Gas Portfolio [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets		5,826		5,826			6,973
Derivative Liability		19,021		19,021			34,926
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets		6,794		6,794			7,566
Derivative Liability		2,241		2,241			4,545
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Electric Portfolio [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets		2,428		2,428			3,525	[2]
Derivative Liability		1,051		1,051			2,427	[2]
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Natural Gas Portfolio [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets		4,366		4,366			4,041	[2]
Derivative Liability		1,190		1,190			2,118	[2]
Carrying Value [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		49,200		49,200			48,500
Valuation, Income Approach [Member] | Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Excluding Current Maturities		5,658,155		5,658,155			5,457,929
Valuation, Income Approach [Member] | Carrying Value [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Subordinated Debt Obligations, Fair Value Disclosure		0		0			250,000
Long-term Debt, Fixed Rate, Net of Discount, Fair Value Disclosure	[3]	5,507,690		5,507,690			5,105,329
Long-term Debt, Variable Rate, Net of Discount, Fair Value Disclosure	[3]	150,465		150,465			102,600
Unamortized Debt Issuance Expense	[3]	26,800		26,800			27,900
Valuation, Income Approach [Member] | Carrying Value [Member] | Subsidiaries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Excluding Current Maturities		3,894,487		3,894,487			3,749,911
Valuation, Income Approach [Member] | Carrying Value [Member] | Subsidiaries [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Subordinated Debt Obligations, Fair Value Disclosure		0		0			250,000
Long-term Debt, Fixed Rate, Net of Discount, Fair Value Disclosure	[3]	3,894,487		3,894,487			3,499,911
Unamortized Debt Issuance Expense	[3]	24,900		24,900			24,600
Valuation, Income Approach [Member] | Total [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Excluding Current Maturities		6,594,680		6,594,680			6,862,050
Valuation, Income Approach [Member] | Total [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Subordinated Debt Obligations, Fair Value Disclosure		0		0			238,935
Long-term Debt, Fixed Rate, Net of Discount, Fair Value Disclosure	[3]	6,444,215		6,444,215			6,520,515
Long-term Debt, Variable Rate, Net of Discount, Fair Value Disclosure	[3]	150,465		150,465			102,600
Valuation, Income Approach [Member] | Total [Member] | Subsidiaries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Excluding Current Maturities		4,564,144		4,564,144			4,789,065
Valuation, Income Approach [Member] | Total [Member] | Subsidiaries [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Subordinated Debt Obligations, Fair Value Disclosure		0		0			238,935
Long-term Debt, Fixed Rate, Net of Discount, Fair Value Disclosure	[3]	$ 4,564,144		$ 4,564,144			$ 4,550,130
Measurement Input, Commodity Forward Price [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Comparability Adjustments (Deprecated 2018-01-31)						0.390

[1] 1 Income Statement locations: Unrealized (gain) loss on derivative instruments, net. Amounts include unrealized gains (losses) on derivatives still held in position as of the reporting date for electric derivatives of $1.4 million and $1.9 million for the nine months ended September 30, 2018 and 2017, respectively.

[2] 1 The valuation techniques, unobservable inputs and ranges are the same for asset and liability positions.

[3] 1 The carrying value includes debt issuances costs of $26.8 million and $27.9 million for September 30, 2018 and December 31, 2017, respectively, which are not included in fair value.