Commitments and Contingencies, Asset Retirement Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	$ 347,800	$ 289,600
Liabilities recognized	12,300	0
Liabilities settled	(7,200)	(24,700)
Accretion	14,700	12,300
Cash Flow Revisions	(28,900)	70,600
Ending balance	338,700	347,800
Electric Plant Steam and Other Production Ash Containment
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	103,200	113,100
Liabilities recognized	0	0
Liabilities settled	(7,100)	(24,100)
Accretion	4,700	5,100
Cash Flow Revisions	1,400	9,100
Ending balance	102,200	103,200
Electric Plant Electric Distribution
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	7,900	7,700
Liabilities recognized	0	0
Liabilities settled	0	0
Accretion	300	200
Cash Flow Revisions	5,200	0
Ending balance	13,400	7,900
Electric Plant Wind Production
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	2,100	2,100
Liabilities recognized	12,300	0
Liabilities settled	0	0
Accretion	100	0
Cash Flow Revisions	0	0
Ending balance	14,500	2,100
Electric Plant Other
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	1,400	1,500
Liabilities recognized	0	0
Liabilities settled	(100)	(200)
Accretion	100	100
Cash Flow Revisions	1,800	0
Ending balance	3,200	1,400
Natural Gas Plant Gas Transmission and Distribution
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	228,900	160,700
Liabilities recognized	0	0
Liabilities settled	0	0
Accretion	9,300	6,700
Cash Flow Revisions	(37,300)	61,500
Ending balance	200,900	228,900
Natural Gas Plant Other
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	3,900	4,100
Liabilities recognized	0	0
Liabilities settled	0	(400)
Accretion	100	200
Cash Flow Revisions	0	0
Ending balance	4,000	3,900
Common and Other Property Common Miscellaneous
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	400	400
Liabilities recognized	0	0
Liabilities settled	0	0
Accretion	100	0
Cash Flow Revisions	0	0
Ending balance	$ 500	$ 400