Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 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 March 31, 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 March 31, 2017
Assets
Coal reclamation trust - cash equivalents (b)	$	14,801	$	—		$	—		$	—			$	14,801
Risk management activities — derivative instruments:
Commodity contracts	—		20,431			9,416			(25,625		)	(c)	4,222
Nuclear decommissioning trust:
U.S. commingled equity funds	—		—			—			374,695			(d)	374,695
Fixed income securities:
Cash and cash equivalent funds	—		—			—			336			(e)	336
U.S. Treasury	94,709		—			—			—				94,709
Corporate debt	—		115,329			—			—				115,329
Mortgage-backed securities	—		115,332			—			—				115,332
Municipal bonds	—		81,932			—			—				81,932
Other	—		22,715			—			—				22,715
Subtotal nuclear decommissioning trust	94,709		335,308			—			375,031				805,048
Total	$	109,510	$	355,739		$	9,416		$	349,406			$	824,071
Liabilities
Risk management activities — derivative instruments:
Commodity contracts	$	—	$	(75,627	)	$	(51,101	)	$	21,583		(c)	$	(105,145	)

(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 March 31, 2017 and December 31, 2016:

	March 31, 2017 Fair Value (thousands)				Valuation Technique	Significant Unobservable Input		Weighted-Average
Commodity Contracts	Assets		Liabilities				Range
Electricity:
Forward Contracts (a)	$	8,805	$	30,313	Discounted cash flows	Electricity forward price (per MWh)	$16.65 - $36.64	$	27.96
Natural Gas:
Forward Contracts (a)	611		20,788		Discounted cash flows	Natural gas forward price (per MMBtu)	$2.07 - $2.80	$	2.42
Total	$	9,416	$	51,101

(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 months ended March 31, 2017 and 2016 (dollars in thousands):

	Three Months Ended   March 31,
Commodity Contracts	2017			2016
Net derivative balance at beginning of period	$	(47,406	)	$	(32,979	)
Total net gains (losses) realized/unrealized:
Included in OCI	—			—
Deferred as a regulatory asset or liability	(11,755		)	(9,103		)
Settlements	1,423			1,765
Transfers into Level 3 from Level 2	(38		)	262
Transfers from Level 3 into Level 2	16,091			548
Net derivative balance at end of period	$	(41,685	)	$	(39,507	)
Net unrealized gains included in earnings related to instruments still held at end of period	$	—		$	—