Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2017

Fair Value Disclosures [Abstract]

Fair value of assets and liabilities that are measured at fair value on a recurring basis

The following table presents the fair value at June 30, 2017, of our assets and liabilities that are measured at fair value on a recurring basis (dollars in thousands):

	Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)			Significant Unobservable Inputs (a) (Level 3)			Other				Balance at June 30, 2017
Assets
Coal reclamation escrow account (b);
Municipal bonds	$	—	$	14,839		$	—		$	—			$	14,839
Risk management activities — derivative instruments:
Commodity contracts	—		10,804			6,440			(16,882		)	(c)	362
Nuclear decommissioning trust:
U.S. commingled equity funds	—		—			—			384,999			(d)	384,999
Fixed income securities:
Cash and cash equivalent funds	—		—			—			1,833			(e)	1,833
U.S. Treasury	101,337		—			—			—				101,337
Corporate debt	—		120,288			—			—				120,288
Mortgage-backed securities	—		113,950			—			—				113,950
Municipal bonds	—		80,659			—			—				80,659
Other	—		19,178			—			—				19,178
Subtotal nuclear decommissioning trust	101,337		334,075			—			386,832				822,244
Total	$	101,337	$	359,718		$	6,440		$	369,950			$	837,445
Liabilities
Risk management activities — derivative instruments:
Commodity contracts	$	—	$	(70,112	)	$	(42,685	)	$	17,598		(c)	$	(95,199	)

(a) Primarily consists of long-dated electricity contracts.

(b) Represents investments restricted for coal mine reclamation funding related to Four Corners. These assets are included in the Other Assets line item, reported under the Investments and Other Assets section of our Condensed Consolidated Balance Sheets.

(c) Represents counterparty netting, margin and collateral. See Note 6.

(d) Valued using NAV as a practical expedient and, therefore, are not classified in the fair value hierarchy.

(e) Represents nuclear decommissioning trust net pending securities sales and purchases.

The following table presents the fair value at December 31, 2016, of our assets and liabilities that are measured at fair value on a recurring basis (dollars in thousands):

	Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)			Significant Unobservable Inputs (a) (Level 3)			Other				Balance at December 31, 2016
Assets
Coal reclamation trust - cash equivalents (b)	$	14,521	$	—		$	—		$	—			$	14,521
Risk management activities — derivative instruments:
Commodity contracts	—		43,722			11,076			(35,103		)	(c)	19,695
Nuclear decommissioning trust:
U.S. commingled equity funds	—		—			—			353,261			(d)	353,261
Fixed income securities:
Cash and cash equivalent funds	—		—			—			795			(e)	795
U.S. Treasury	95,441		—			—			—				95,441
Corporate debt	—		111,623			—			—				111,623
Mortgage-backed securities	—		115,337			—			—				115,337
Municipal bonds	—		80,997			—			—				80,997
Other	—		22,132			—			—				22,132
Subtotal nuclear decommissioning trust	95,441		330,089			—			354,056				779,586
Total	$	109,962	$	373,811		$	11,076		$	318,953			$	813,802
Liabilities
Risk management activities — derivative instruments:
Commodity contracts	$	—	$	(45,641	)	$	(58,482	)	$	31,049		(c)	$	(73,074	)

(a) Primarily consists of long-dated electricity contracts.

(b) Represents investments restricted for coal mine reclamation funding related to Four Corners. These assets are included in the Other Assets line item, reported under the Investments and Other Assets section of our Condensed Consolidated Balance Sheets.

(c) Represents counterparty netting, margin and collateral. See Note 6.

(d) Valued using NAV as a practical expedient and, therefore, are not classified in the fair value hierarchy.

(e) Represents nuclear decommissioning trust net pending securities sales and purchases.

Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments

The following tables provide information regarding our significant unobservable inputs used to value our risk management derivative Level 3 instruments at June 30, 2017 and December 31, 2016:

	June 30, 2017 Fair Value (thousands)				Valuation Technique	Significant Unobservable Input		Weighted-Average
Commodity Contracts	Assets		Liabilities				Range
Electricity:
Forward Contracts (a)	$	5,996	$	25,514	Discounted cash flows	Electricity forward price (per MWh)	$18.46 - $38.43	$	28.48
Natural Gas:
Forward Contracts (a)	444		17,171		Discounted cash flows	Natural gas forward price (per MMBtu)	$2.16 - $2.81	$	2.50
Total	$	6,440	$	42,685

(a) Includes swaps and physical and financial contracts.

	December 31, 2016 Fair Value (thousands)				Valuation Technique	Significant Unobservable Input		Weighted-Average
Commodity Contracts	Assets		Liabilities				Range
Electricity:
Forward Contracts (a)	$	10,648	$	32,042	Discounted cash flows	Electricity forward price (per MWh)	$16.43 - $41.07	$	29.86
Natural Gas:
Forward Contracts (a)	428		26,440		Discounted cash flows	Natural gas forward price (per MMBtu)	$2.32 - $3.60	$	2.81
Total	$	11,076	$	58,482

(a) Includes swaps and physical and financial contracts.

Changes in fair value for assets and liabilities that are measured at fair value on a recurring basis using Level 3 inputs

The following table shows the changes in fair value for our risk management activities' assets and liabilities that are measured at fair value on a recurring basis using Level 3 inputs for the three and six months ended June 30, 2017 and 2016 (dollars in thousands):

	Three Months Ended   June 30,						Six Months Ended   June 30,
Commodity Contracts	2017			2016			2017			2016
Net derivative balance at beginning of period	$	(41,685	)	$	(39,507	)	$	(47,406	)	$	(32,979	)
Total net gains (losses) realized/unrealized:
Included in OCI	(6		)	104			(6		)	104
Deferred as a regulatory asset or liability	4,252			1,499			(7,503		)	(7,604		)
Settlements	1,699			4,502			3,122			6,267
Transfers into Level 3 from Level 2	(4,350		)	120			(4,388		)	382
Transfers from Level 3 into Level 2	3,845			902			19,936			1,450
Net derivative balance at end of period	$	(36,245	)	$	(32,380	)	$	(36,245	)	$	(32,380	)
Net unrealized gains included in earnings related to instruments still held at end of period	$	—		$	—		$	—		$	—