The following table presents our regulatory assets and regulatory liabilities at December 31, 2016 and 2015:



	2016						2015
	Ameren Missouri		Ameren Illinois		Ameren			Ameren Missouri		Ameren Illinois		Ameren
Current regulatory assets:
Under-recovered FAC^(a)(b)	$	21	$	—	$	21		$	37	$	—	$	37
Under-recovered Illinois electric power costs^(c)	—		3		3			—		3		3
Under-recovered PGA^(c)	—		4		4			—		8		8
MTM derivative losses^(d)	9		15		24			29		45		74
Energy efficiency riders^(e)	5		—		5			23		—		23
IEIMA revenue requirement reconciliation adjustment^(a)(f)	—		68		68			—		103		103
FERC revenue requirement reconciliation adjustment^(a)(g)	—		7		13			—		8		12
VBA rider^(a)(h)	—		11		11			—		—		—
Total current regulatory assets	$	35	$	108	$	149		$	89	$	167	$	260
Noncurrent regulatory assets:
Pension and postretirement benefit costs⁽ⁱ⁾	$	175	$	319	$	494		$	95	$	202	$	297
Income taxes^(j)	229		1		230			247		4		251
Uncertain tax positions tracker^(a)(k)	7		—		7			7		—		7
ARO^(l)	—		3		3			—		4		4
Callaway costs^(a)(m)	29		—		29			32		—		32
Unamortized loss on reacquired debt^(a)(n)	65		59		124			69		69		138
Environmental cost riders^(o)	—		196		196			—		230		230
MTM derivative losses^(d)	9		178		187			15		175		190
Storm costs^(a)(p)	—		15		15			—		9		9
Demand-side costs before the MEEIA implementation^(a)(q)	18		—		18			31		—		31
Workers’ compensation claims^(r)	6		7		13			6		7		13
Credit facilities fees^(s)	4		—		4			4		—		4
Construction accounting for pollution control equipment^(a)(t)	19		—		19			20		—		20
Solar rebate program^(a)(u)	49		—		49			74		—		74
IEIMA revenue requirement reconciliation adjustment^(a)(f)	—		23		23			—		62		62
FERC revenue requirement reconciliation adjustment^(a)(g)	—		8		10			—		5		11
Other	9		7		16			5		4		9
Total noncurrent regulatory assets	$	619	$	816	$	1,437		$	605	$	771	$	1,382
Current regulatory liabilities:
Over-recovered FAC^(b)	$	—	$	—	$	—		$	9	$	—	$	9
Over-recovered Illinois electric power costs^(c)	—		25		25			—		6		6
Over-recovered PGA^(c)	—		—		—			3		—		3
MTM derivative gains^(d)	12		11		23			16		1		17
Estimated refund for FERC complaint cases^(v)	—		42		62			—		32		45
Total current regulatory liabilities	$	12	$	78	$	110		$	28	$	39	$	80
Noncurrent regulatory liabilities:
Income taxes^(w)	$	33	$	4	$	37		$	36	$	6	$	42
Uncertain tax positions tracker^(k)	3		—		3			6		—		6
Asset removal costs^(x)	970		697		1,669			933		671		1,605
ARO^(l)	162		—		162			167		—		167
Bad debt rider^(y)	—		3		3			—		6		6
Pension and postretirement benefit costs tracker^(z)	35		—		35			19		—		19
Energy efficiency riders^(e)	—		45		45			—		36		36
Renewable energy credits^(aa)	—		15		15			—		12		12
Storm tracker^(ab)	7		—		7			9		—		9
Other	5		4		9			2		1		3
Total noncurrent regulatory liabilities	$	1,215	$	768	$	1,985		$	1,172	$	732	$	1,905


(a)	These assets earn a return.


(b)	Under-recovered or over-recovered fuel costs to be recovered or refunded through the FAC. Specific accumulation periods aggregate the under-recovered or over-recovered costs over four months, any related adjustments that occur over the following four months, and the recovery from or refund to customers that occurs over the next eight months.


(c)	Under-recovered or over-recovered costs from utility customers. Amounts will be recovered from or refunded to customers within one year of the deferral.


(d)	Deferral of commodity-related derivative MTM losses or gains. See Note 7 – Derivative Financial Instruments for additional information.

(e)

The Ameren Missouri balance relates to the MEEIA. The MEEIA rider allows Ameren Missouri to collect from or refund to customers any annual difference in the actual amounts incurred and the amounts collected from customers for the MEEIA program costs, net shared benefits, and the throughput disincentive. Under the MEEIA rider, collections from or refunds to customers occur one year after the program costs, net shared benefits, and the throughput disincentive are incurred. The Ameren Illinois balance relates to a regulatory tracking mechanism to recover its electric and natural gas costs associated with developing, implementing, and evaluating customer energy efficiency and demand response programs. Any under-recovery or over-recovery will be collected from or refunded to customers over the year following the plan year.


(f)	The difference between Ameren Illinois' annual revenue requirement calculated under the IEIMA's performance-based formula ratemaking framework and the revenue requirement included in customer rates for that year. The under-recovery will be recovered from or refunded to customers with interest within two years.


(g)	Ameren Illinois' and ATXI's annual revenue requirement reconciliation calculated pursuant to the FERC's electric transmission formula ratemaking framework. The under-recovery or over-recovery will be recovered from or refunded to customers within two years.


(h)	Under-recovered natural gas sales volumes, including deviations from normal weather conditions. Each year's amount will be recovered from or refunded to customers from April through December of the following year.


(i)	These costs are being amortized in proportion to the recognition of prior service costs (credits) and actuarial losses (gains) attributable to Ameren’s pension plan and postretirement benefit plans. See Note 11 – Retirement Benefits for additional information.


(j)	Tax benefits related to the equity component of allowance for funds used during construction, as well as the effects of tax rate changes. This amount will be recovered over the expected life of the related assets.


(k)	The tracker is amortized over three years, beginning from the date the amounts are included in rates. See Note 13 – Income Taxes for additional information.


(l)	Recoverable or refundable removal costs for AROs, including net realized and unrealized gains and losses related to the nuclear decommissioning trust fund investments. See Note 1 – Summary of Significant Accounting Policies – Asset Retirement Obligations.


(m)	Ameren Missouri’s Callaway energy center operations and maintenance expenses, property taxes, and carrying costs incurred between the plant in-service date and the date the plant was reflected in rates. These costs are being amortized over the remaining life of the energy center's original operating license through 2024.


(n)	Losses related to reacquired debt. These amounts are being amortized over the lives of the related new debt issuances or the original lives of the old debt issuances if no new debt was issued.


(o)	The recoverable portion of accrued environmental site liabilities that will be collected from electric and natural gas customers through ICC-approved cost recovery riders. The period of recovery will depend on the timing of remediation expenditures. See Note 15 – Commitments and Contingencies for additional information.


(p)	Storm costs from 2013, 2015, and 2016 deferred in accordance with the IEIMA. These costs are being amortized over five-year periods beginning in the year the storm occurred.

(q)

Demand-side costs incurred prior to implementation of the MEEIA in 2013, including the costs of developing, implementing, and evaluating customer energy efficiency and demand response programs. Costs incurred from May 2008 through September 2008 are being amortized over a 10-year period that began in March 2009. Costs incurred from October 2008 through December 2009 are being amortized until May 2017. Costs incurred from January 2010 through February 2011 are being amortized over a six-year period that began in August 2011. Costs incurred from March 2011 through July 2012 are being amortized over a six-year period that began in January 2013. Costs incurred from August 2012 through December 2012 are being amortized over a six-year period that began in June 2015. The February 2017 stipulation and agreement, if approved, would modify these amortization periods.


(r)	The period of recovery will depend on the timing of actual expenditures.


(s)	Ameren Missouri’s costs incurred to enter into and maintain the Missouri Credit Agreement. These costs are being amortized over the life of the credit facility to construction work in progress, which will be depreciated when assets are placed into service. Additional costs were incurred in December 2016 to amend and restate the Missouri Credit Agreement.


(t)	The MoPSC’s May 2010 electric rate order allowed Ameren Missouri to record an allowance for funds used during construction for pollution control equipment at its Sioux energy center until the cost of that equipment was included in customer rates beginning in 2011. These costs are being amortized over the expected life of the Sioux energy center, currently through 2033.


(u)	Costs associated with Ameren Missouri's solar rebate program to fulfill its renewable energy portfolio requirement. These costs are being amortized over a three-year period that began in June 2015. The February 2017 stipulation and agreement, if approved, would modify this amortization period.


(v)	Estimated refunds to transmission customers related to FERC orders. See FERC Complaint Cases above.


(w)	Unamortized portion of investment tax credits and reductions to deferred tax liabilities recorded at rates in excess of current statutory rates. The unamortized portion of investment tax credits and the reduction to deferred tax liabilities are being amortized over the expected life of the underlying assets.


(x)	Estimated funds collected for the eventual dismantling and removal of plant retired from service, net of salvage value.

(y)

A regulatory tracking mechanism for the difference between the level of bad debt incurred by Ameren Illinois under GAAP and the level of such costs included in electric and natural gas rates. The over-recovery relating to 2014 was refunded to customers from June 2015 through May 2016. The over-recovery relating to 2015 is being refunded to customers from June 2016 through May 2017. The over-recovery relating to 2016 will be refunded to customers from June 2017 through May 2018.

(z)

A regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. For periods prior to December 2014, the MoPSC's April 2015 electric rate order directed the amortization to occur over three to five years, beginning in June 2015. For periods after December 2014, the amortization period will be determined in the July 2016 electric rate case. The February 2017 stipulation and agreement, if approved, would modify these amortization periods.


(aa)	Funds collected from customers for the purchase of renewable energy credits through IPA procurements for distributed generation. The balance will be amortized as renewable energy credits are purchased.

(ab)

A regulatory tracking mechanism at Ameren Missouri for the difference between the level of storm costs incurred in a particular year and the level of such costs included in rates. For periods prior to December 2014, the MoPSC's April 2015 electric rate order directed the amortization to occur over a five-year period that began in June 2015. For periods after December 2014, the amortization period will be determined in the July 2016 electric rate case. The April 2015 MoPSC order did not approve the continued use of the storm cost regulatory tracking mechanism.