Fair Value Measurements - Unobservable Input Reconciliation (Details) (USD $)	12 Months Ended						6 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Jun. 30, 2013	Jul. 01, 2013	Mar. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Finite-lived Intangible Assets, Fair Value Disclosure	$ 437,600,000							$ 484,100,000
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	(15,782,000)		(35,526,000)		(95,352,000)		(35,526,000)
Included in earnings	(5,537,000)	[1]	(10,491,000)	[1]	(21,362,000)	[1]
Included in regulatory assets / liabilities	1,630,000		(945,000)		(1,937,000)
Settlements	(498,000)	[2]	10,855,000	[2]	60,102,000	[2]
Transferred into Level 3	4,570,000		(7,799,000)		(55,845,000)
Transferred out of Level 3	1,515,000		28,124,000		78,868,000
Balance at end of period	(14,102,000)		(15,782,000)		(35,526,000)
Electric Portfolio
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	(15,421,000)		(33,924,000)		(90,311,000)		(33,924,000)
Included in earnings	(5,537,000)	[1]	(10,491,000)	[1]	(21,362,000)	[1]
Included in regulatory assets / liabilities	0		0		0
Settlements	1,036,000	[2]	11,609,000	[2]	59,133,000	[2]
Transferred into Level 3	5,155,000		(7,799,000)		(55,548,000)
Transferred out of Level 3	2,705,000		25,184,000		74,164,000
Balance at end of period	(12,062,000)		(15,421,000)		(33,924,000)
Unrealized gain (loss) on derivative instruments, net	(9,600,000)		(13,400,000)		(15,200,000)
Natural Gas Portfolio
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	(361,000)		(1,602,000)		(5,041,000)		(1,602,000)
Included in earnings	0	[1]	0	[1]	0	[1]
Included in regulatory assets / liabilities	1,630,000		(945,000)		(1,937,000)
Settlements	(1,534,000)	[2]	(754,000)	[2]	969,000	[2]
Transferred into Level 3	(585,000)		0		(297,000)
Transferred out of Level 3	(1,190,000)		2,940,000		4,704,000
Balance at end of period	(2,040,000)		(361,000)		(1,602,000)
Wells Project
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Finite-Lived Intangible Assets, Net	65,300,000
Reported Value Measurement [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Finite-lived Intangible Assets, Fair Value Disclosure	62,100,000							33,600,000
Portion at Fair Value Measurement [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Finite-lived Intangible Assets, Fair Value Disclosure								13,500,000	3,200,000
Income Approach Valuation Technique | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Price Per Megawatt-Hour	19.30						30.85
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value Inputs, Power Contract Costs	3,015,000						389,000
Income Approach Valuation Technique | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Price Per Megawatt-Hour	37.06						65.35
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value Inputs, Power Contract Costs	4,783,000						6,845,000
Income Approach Valuation Technique | Weighted Average
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Price Per Megawatt-Hour	29.53						48.47
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value Inputs, Power Contract Costs	$ 4,469,000						$ 4,110,000

[1] Income Statement location: Unrealized (gain) loss on derivative instruments, net. Includes unrealized gains (losses) on derivatives still held in position as of the reporting date for electric derivatives of $(9.6) million, $(13.4) million, and $(15.2) million for the years ended December 31, 2014, 2013 and 2012, respectively.

[2] The Company had no purchases, sales or issuances during the reported periods.