Accounting for Derivative Instruments and Hedging Activities Net Amount of Derivatives Reported in the Statement of Financial Position (Details) $ in Thousands, MMBTU in Millions	Sep. 30, 2018 USD ($) MMBTU		Dec. 31, 2017 USD ($) MMBTU
Derivative [Line Items]
Derivative Asset, Current	$ 16,524		$ 22,247
Derivative Liability, Current	30,558		64,859
Derivative Asset, Noncurrent	1,897		2,158
Derivative Liability, Noncurrent	11,588		21,235
Commodity Contract [Member]
Assets:
Gross Amount Recognized in the Statement of Financial Position	18,421	[1]	24,405
Gross Amounts Offset in the Statement of Financial Position	0		0
Net of Amounts Presented in the Statement of Financial Position	18,421		24,405
Commodity Contracts	(11,058)		(17,940)
Cash Collateral Received	0		0
Net Amount	7,363		6,465
Liabilities:
Gross Amount Recognized in the Statement of Financial Position	42,146	[1]	86,094
Gross Amounts Offset in the Statement of Financial Position	0		0
Net of Amounts Presented in the Statement of Financial Position	42,146		86,094
Commodity Contracts	(11,058)		(17,940)
Cash Collateral Posted	0		(353)
Net Amount	31,088		67,801
Subsidiaries [Member]
Derivative [Line Items]
Derivative Asset, Current	16,524		22,247
Derivative Liability, Current	30,558		64,859
Derivative Asset, Noncurrent	1,897		2,158
Derivative Liability, Noncurrent	11,588		21,235
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset, Current	16,524		22,247
Derivative Liability, Current	30,558		64,859
Derivative Asset, Noncurrent	1,897		2,158
Derivative Liability, Noncurrent	11,588		21,235
Assets:
Net of Amounts Presented in the Statement of Financial Position	18,421		24,405
Liabilities:
Net of Amounts Presented in the Statement of Financial Position	42,146		86,094
Not Designated as Hedging Instrument [Member] | Electric Portfolio [Member]
Assets:
Net of Amounts Presented in the Statement of Financial Position	8,229		13,391
Liabilities:
Net of Amounts Presented in the Statement of Financial Position	$ 21,935		$ 49,050
Not Designated as Hedging Instrument [Member] | Natural Gas Derivatives [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount | MMBTU	302.0		332.1
Not Designated as Hedging Instrument [Member] | Natural Gas Portfolio [Member]
Assets:
Net of Amounts Presented in the Statement of Financial Position	$ 10,192		$ 11,014
Liabilities:
Net of Amounts Presented in the Statement of Financial Position	$ 20,211		$ 37,044

[1] 1 All derivative contract deals are executed under ISDA, NAESB and WSPP master netting agreements with right of set-off.