4. Retirement Plans and Other Benefits

Pinnacle West sponsors a qualified defined benefit and account balance pension plan, a non-qualified supplemental excess benefit retirement plan, and other postretirement benefit plans for the employees of Pinnacle West and our subsidiaries. Pinnacle West uses a December 31 measurement date for its pension and other postretirement benefit plans. The market-related value of our plan assets is their fair value at the measurement date.

Certain pension and other postretirement benefit costs in excess of amounts recovered in electric retail rates were deferred through June 30, 2012 as a regulatory asset for future recovery, pursuant to an ACC regulatory order. We deferred pension and other postretirement benefit costs of approximately $14 million in 2012. Pursuant to an ACC regulatory order, we began amortizing the regulatory asset in July 2012. We amortized approximately $2 million for the three months ended September 30, 2013, and 2012, and approximately $6 million and $2 million for the nine months ended September 30, 2013 and 2012, respectively. The following table provides details of the plans’ net periodic benefit costs and the portion of these costs charged to expense (including administrative costs and excluding amounts capitalized as overhead construction, billed to electric plant participants or charged or amortized to the regulatory asset) (dollars in millions):

	Pension Benefits												Other Benefits
	Three Months Ended September 30,						Nine Months Ended September 30,						Three Months Ended September 30,						Nine Months Ended September 30,
	2013			2012			2013			2012			2013			2012			2013			2012
Service cost — benefits earned during the period	$	16		$	16		$	48		$	48		$	6		$	7		$	18		$	20
Interest cost on benefit obligation	28			30			84			90			10			12			31			35
Expected return on plan assets	(36		)	(35		)	(110		)	(106		)	(11		)	(12		)	(34		)	(34		)
Amortization of:
Prior service cost	—			—			1			1			—			—			—			—
Net actuarial loss	10			11			30			33			3			5			8			15
Net periodic benefit cost	$	18		$	22		$	53		$	66		$	8		$	12		$	23		$	36
Portion of cost charged to expense	$	10		$	12		$	29		$	25		$	5		$	7		$	14		$	13

Contributions

We have made voluntary contributions of $141 million to our pension plan in 2013. The minimum contributions for the pension plan due in 2013, 2014, and 2015 under the recently enacted Moving Ahead for Progress in the 21st Century Act (MAP-21) are estimated to be zero, $89 million, and $112 million, respectively. We expect to make contributions to the pension plan up to approximately $175 million each year in 2014 and 2015. We have contributed $11 million to our other postretirement benefit plans in 2013. The total contributions to our other postretirement benefit plans are expected to be approximately $14 million in 2013 and approximately $20 million each year in 2014 and 2015.