Investments (Tables)

12 Months Ended

Dec. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Fair value of APS's nuclear decommissioning trust fund assets

The following table includes the unrealized gains and losses based on the original cost of the investment and summarizes the fair value of APS’s nuclear decommissioning trust fund assets at December 31, 2017 and December 31, 2016 (dollars in thousands):

	December 31, 2017							December 31, 2016
	Fair Value		Total  Unrealized  Gains		Total  Unrealized  Losses			Fair Value		Total  Unrealized  Gains		Total  Unrealized  Losses
Equity securities	$	417,390	$	248,623	$	—		$	353,261	$	188,091	$	—
Fixed income securities	446,277		11,537		(2,996		)	425,530		9,820		(4,962		)
Cash and cash equivalents	7,224		—		—			—		—		—
Net receivables (a)	109		—		—			795		—		—
Total	$	871,000	$	260,160	$	(2,996	)	$	779,586	$	197,911	$	(4,962	)

(a) Net receivables/(payables) relate to pending purchases and sales of securities.

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

The following table sets forth approximate gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds (dollars in thousands):

	Nuclear Decommissioning
	Year Ended December 31,
	2017			2016			2015
Realized gains	$	21,813		$	11,213		$	5,189
Realized losses	(13,146		)	(10,106		)	(6,225		)
Proceeds from the sale of securities (a)	542,246			633,410			478,813

(a) Proceeds are reinvested in the trust/account.

Fair value of fixed income securities, summarized by contractual maturities

The fair value of fixed income securities, summarized by contractual maturities, at December 31, 2017 is as follows (dollars in thousands):

	Nuclear Decommissioning Trusts		Escrow Accounts		Total
Less than one year	$	24,668	$	455	$	25,123
1 year – 5 years	100,289		2,494		102,783
5 years – 10 years	129,239		8,615		137,854
Greater than 10 years	192,081		20,453		212,534
Total	$	446,277	$	32,017	$	478,294