Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Tables)

6 Months Ended

Jun. 30, 2018

Investments, Debt and Equity Securities [Abstract]

Fair value of APS's nuclear decommissioning trust fund assets

The following tables present the unrealized gains and losses based on the original cost of the investment and summarizes the fair value of APS's nuclear decommissioning trust and other special use fund assets at June 30, 2018 and December 31, 2017 (dollars in thousands):  

	June 30, 2018
	Fair Value							Total Unrealized Gains		Total Unrealized Losses
Investment Type:	Nuclear Decommissioning Trusts		Other Special Use Funds		Total
Equity securities	$	432,615	$	14,109	$	446,724		$	254,342	$	(1	)
Available for sale-fixed income securities	440,403		200,997		641,400		(a)	6,896		(9,804		)
Other	625		1,232		1,857		(b)	—		—
Total	$	873,643	$	216,338	$	1,089,981		$	261,238	$	(9,805	)

(a) As of June 30, 2018, the amortized cost basis of these available-for-sale investments is $639 million.

(b) Represents net pending securities sales and purchases.

	December 31, 2017
	Fair Value							Total Unrealized Gains		Total Unrealized Losses
Investment Type:	Nuclear Decommissioning Trusts		Other Special Use Funds		Total
Equity securities	$	424,614	$	430	$	425,044		$	248,623	$	—
Available for sale-fixed income securities	446,277		29,439		475,716		(a)	11,537		(2,996		)
Other	109		489		598		(b)	—		—
Total	$	871,000	$	30,358	$	901,358		$	260,160	$	(2,996	)

(a) As of December 31, 2017, the amortized cost basis of these available-for-sale investments is $467 million.

(b) Represents net pending securities sales and purchases.

Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds

The following table sets forth APS's realized gains and losses relating to the sale and maturity of available-for-sale debt securities and equity securities, and the proceeds from the sale and maturity of these investment securities for the three and six months ended June 30, 2018 and June 30, 2017 (dollars in thousands):

	Three Months Ended   June 30, 2018								Three Months Ended   June 30, 2017
	Nuclear Decommissioning Trusts			Other Special Use Funds		Total			Nuclear Decommissioning Trusts			Other Special Use Funds			Total
Realized gains	$	1,484		$	—	$	1,484		$	939		$	17		$	956
Realized losses	(2,978		)	—		(2,978		)	(1,159		)	(8		)	(1,167		)
Proceeds from the sale of securities (a)	122,790			2,426		125,216			124,238			1,572			125,810

(a)    Proceeds are reinvested in the trust or escrow accounts.

	Six Months Ended   June 30, 2018								Six Months Ended   June 30, 2017
	Nuclear Decommissioning Trusts			Other Special Use Funds		Total			Nuclear Decommissioning Trusts			Other Special Use Funds			Total
Realized gains	$	2,298		$	1	$	2,299		$	3,306		$	17		$	3,323
Realized losses	(5,025		)	—		(5,025		)	(3,612		)	(9		)	(3,621		)
Proceeds from the sale of securities (a)	253,246			4,981		258,227			275,364			4,093			279,457

(a)    Proceeds are reinvested in the trust or escrow accounts.

Fair value of fixed income securities, summarized by contractual maturities

     The fair value of APS's fixed income securities, summarized by contractual maturities, at June 30, 2018, is as follows (dollars in thousands):

	Nuclear Decommissioning Trusts (a)		Coal Reclamation Escrow Accounts		Active Union Medical Trust		Total
Less than one year	$	18,415	$	—	$	30,441	$	48,856
1 year – 5 years	105,470		11,706		143,197		260,373
5 years – 10 years	127,599		2,715		—		130,314
Greater than 10 years	188,919		12,938		—		201,857
Total	$	440,403	$	27,359	$	173,638	$	641,400

(a) Includes certain fixed income investments that are not due at a single maturity date. These investments have been allocated within the table based on the final payment date of the instrument.