Fair Value Measurements Assets and Liabilities (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Excluding Current Maturities		$ 5,645,191		$ 5,645,191		$ 5,257,929
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		6,282	$ 5,540	3,021	$ 1,597
Included in earnings	[1]	366	339	1,985	1,045
Included in regulatory assets/liabilities		354	1,124	5,329	3,582
Settlements		(1,951)	(4,482)	(4,256)	(7,142)
Transferred into Level 3		0	0	(1,837)	1,638
Transferred out of Level 3		907	(422)	1,716	1,379
Balance at end of period		5,958	2,099	5,958	2,099
Electric Portfolio [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts		700	500	1,700	700
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		1,186	3,788	1,098	972
Included in earnings	[1]	366	339	1,985	1,045
Included in regulatory assets/liabilities		0	0	0	0
Settlements		(151)	(2,508)	(654)	(3,838)
Transferred into Level 3		0	0	(1,837)	2,191
Transferred out of Level 3		608	(976)	1,417	273
Balance at end of period		2,009	643	2,009	643
Natural Gas Portfolio [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		5,096	1,752	1,923	625
Included in earnings	[1]	0	0	0	0
Included in regulatory assets/liabilities		354	1,124	5,329	3,582
Settlements		(1,800)	(1,974)	(3,602)	(3,304)
Transferred into Level 3		0	0	0	(553)
Transferred out of Level 3		299	554	299	1,106
Balance at end of period		3,949	$ 1,456	3,949	$ 1,456
Subsidiaries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Excluding Current Maturities		3,895,042		3,895,042		3,549,911
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets		23,461		23,461		24,405
Derivative Liability		64,899		64,899		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		10,889		10,889		13,391
Derivative Liability		38,642		38,642		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		12,572		12,572		11,014
Derivative Liability		26,257		26,257		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		13,739		13,739		16,839
Derivative Liability		61,135		61,135		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		7,229		7,229		9,866
Derivative Liability		36,991		36,991		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		6,510		6,510		6,973
Derivative Liability		24,144		24,144		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		9,722		9,722		7,566
Derivative Liability		3,764		3,764		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		3,660		3,660		3,525	[2]
Derivative Liability		1,651		1,651		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		6,062		6,062		4,041	[2]
Derivative Liability		2,113		2,113		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		49,100		49,100		48,500
Income Approach Valuation Technique [Member] | Carrying Value [Member] | Parent Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Excluding Current Maturities		5,645,191		5,645,191		5,457,929
Income Approach Valuation Technique [Member] | Carrying Value [Member] | Parent Company [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]	5,505,640		5,505,640		5,105,329
Long-term Debt, Variable Rate, Net of Discount, Fair Value Disclosure	[3]	139,551		139,551		102,600
Unamortized Debt Issuance Expense	[3]	26,600		26,600		27,900
Income Approach Valuation Technique [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		3,895,042		3,895,042		3,749,911
Income Approach Valuation Technique [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]	3,895,042		3,895,042		3,499,911
Unamortized Debt Issuance Expense	[3]	24,200		24,200		24,600
Income Approach Valuation Technique [Member] | Total [Member] | Parent Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Excluding Current Maturities		6,623,270		6,623,270		6,862,050
Income Approach Valuation Technique [Member] | Total [Member] | Parent Company [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]	6,483,719		6,483,719		6,520,515
Long-term Debt, Variable Rate, Net of Discount, Fair Value Disclosure	[3]	139,551		139,551		102,600
Income Approach Valuation Technique [Member] | Total [Member] | Subsidiaries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Excluding Current Maturities		4,598,167		4,598,167		4,789,065
Income Approach Valuation Technique [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]	$ 4,598,167		$ 4,598,167		$ 4,550,130

[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 $0.7 million and $0.5 million for the three months ended June 30, 2018 and 2017, respectively. 2 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 $1.7 million and $0.7 million for the six months ended June 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 $26.6 million and $27.9 million for June 30, 2018 and December 31, 2017, respectively, which are not included in fair value.