Fair Value Measurements Assets and Liabilities (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Excluding Current Maturities		$ 5,080,669		$ 5,257,929
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		3,021	$ 1,597
Included in earnings	[1]	1,619	706
Included in regulatory assets/liabilities		4,976	2,458
Settlements		(2,306)	(2,659)
Transferred into Level 3		(1,837)	1,635
Transferred out of Level 3		809	1,803
Balance at end of period		6,282	5,540
Electric Portfolio [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts		2,000	1,800
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		1,098	972
Included in earnings	[1]	1,619	706
Included in regulatory assets/liabilities		0	0
Settlements		(503)	(1,330)
Transferred into Level 3		(1,837)	2,189
Transferred out of Level 3		809	1,251
Balance at end of period		1,186	3,788
Natural Gas Portfolio [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		1,923	625
Included in earnings	[1]	0	0
Included in regulatory assets/liabilities		4,976	2,458
Settlements		(1,803)	(1,329)
Transferred into Level 3		0	(554)
Transferred out of Level 3		0	552
Balance at end of period		5,096	$ 1,752
Subsidiaries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Excluding Current Maturities		3,356,884		3,549,911
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets		27,161		24,405
Derivative Liability		84,080		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		13,150		13,391
Derivative Liability		47,813		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		14,011		11,014
Derivative Liability		36,267		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		14,475		16,839
Derivative Liability		77,676		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		8,483		9,866
Derivative Liability		44,332		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		5,992		6,973
Derivative Liability		33,344		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		12,686		7,566
Derivative Liability		6,404		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		4,667		3,525	[2]
Derivative Liability		3,481		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		8,019		4,041	[2]
Derivative Liability		2,923		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		48,600		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		5,280,669		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		56,553		250,000
Long-term Debt, Fixed Rate, Net of Discount, Fair Value Disclosure	[3]	5,108,337		5,105,329
Long-term Debt, Variable Rate, Net of Discount, Fair Value Disclosure	[3]	115,779		102,600
Unamortized Debt Issuance Expense	[3]	27,000		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		3,556,884		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		56,553		250,000
Long-term Debt, Fixed Rate, Net of Discount, Fair Value Disclosure	[3]	3,500,331		3,499,911
Unamortized Debt Issuance Expense	[3]	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		6,362,919		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		53,319		238,935
Long-term Debt, Fixed Rate, Net of Discount, Fair Value Disclosure	[3]	6,193,821		6,520,515
Long-term Debt, Variable Rate, Net of Discount, Fair Value Disclosure	[3]	115,779		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		4,344,771		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		53,319		238,935
Long-term Debt, Fixed Rate, Net of Discount, Fair Value Disclosure	[3]	$ 4,291,452		$ 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 $2.0 million and $1.8 million for the three months ended March 31, 2018 and 2017.

[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 $27.0 million and $27.9 million for March 31, 2018 and December 31, 2017, respectively, which are not included in fair value.