The following table presents our regulatory assets and regulatory liabilities at December 31, 2017 and 2016:



	2017						2016
	Ameren Missouri		Ameren Illinois		Ameren			Ameren Missouri		Ameren Illinois		Ameren
Current regulatory assets:
Under-recovered FAC^(a)(b)	$	47	$	—	$	47		$	21	$	—	$	21
Under-recovered Illinois electric power costs^(c)	—		—		—			—		3		3
Under-recovered PGA^(c)	1		13		14			—		4		4
MTM derivative losses^(d)	8		25		33			9		15		24
Energy-efficiency riders^(e)	—		—		—			5		—		5
IEIMA revenue requirement reconciliation adjustment^(a)(f)	—		24		24			—		68		68
FERC revenue requirement reconciliation adjustment^(a)(g)	—		9		10			—		7		13
VBA rider^(a)(h)	—		15		15			—		11		11



	2017						2016
	Ameren Missouri		Ameren Illinois		Ameren			Ameren Missouri		Ameren Illinois		Ameren
Other	—		1		1			—		—		—
Total current regulatory assets	$	56	$	87	$	144		$	35	$	108	$	149
Noncurrent regulatory assets:
Pension and postretirement benefit costs⁽ⁱ⁾	$	84	$	215	$	299		$	175	$	319	$	494
Income taxes^(j)	139		56		197			229		1		230
Uncertain tax positions tracker^(a)(k)	5		—		5			7		—		7
ARO^(l)	—		1		1			—		3		3
Callaway costs^(a)(m)	25		—		25			29		—		29
Unamortized loss on reacquired debt^(a)(n)	61		49		110			65		59		124
Environmental cost riders^(o)	—		173		173			—		196		196
MTM derivative losses^(d)	4		192		196			9		178		187
Storm costs^(a)(p)	—		10		10			—		15		15
Demand-side costs before the MEEIA implementation^(a)(q)	11		—		11			18		—		18
Workers’ compensation claims^(r)	5		7		12			6		7		13
Credit facilities fees^(s)	3		—		3			4		—		4
Construction accounting for pollution control equipment^(a)(t)	18		—		18			19		—		19
Solar rebate program^(a)(u)	31		—		31			49		—		49
IEIMA revenue requirement reconciliation adjustment^(a)(f)	—		54		54			—		23		23
FERC revenue requirement reconciliation adjustment^(a)(g)	—		16		27			—		8		10
FEJA energy-efficiency riders^(a)(v)	—		41		41			—		—		—
Other	9		8		17			9		7		16
Total noncurrent regulatory assets	$	395	$	822	$	1,230		$	619	$	816	$	1,437
Current regulatory liabilities:
Over-recovered FAC^(b)	$	4	$	—	$	4		$	—	$	—	$	—
Over-recovered Illinois electric power costs^(c)	—		16		16			—		25		25
Over-recovered PGA^(c)	—		1		1			—		—		—
MTM derivative gains^(d)	13		—		13			12		11		23
Energy-efficiency riders^(e)	2		40		42			—		—		—
Estimated refund for FERC complaint case^(w)	—		25		42			—		42		62
Other	—		10		10			—		—		—
Total current regulatory liabilities	$	19	$	92	$	128		$	12	$	78	$	110
Noncurrent regulatory liabilities:
Income taxes^(j)	$	1,392	$	842	$	2,323		$	33	$	4	$	37
Uncertain tax positions tracker^(k)	2		—		2			3		—		3
Asset removal costs^(x)	995		725		1,725			970		697		1,669
ARO^(l)	223		—		223			162		—		162
Bad debt rider^(y)	—		2		2			—		3		3
Pension and postretirement benefit costs tracker^(z)	35		—		35			35		—		35
Energy-efficiency riders^(e)	—		—		—			—		45		45
Renewable energy credits and zero-emission credits^(aa)	—		58		58			—		15		15
Storm tracker^(ab)	6		—		6			7		—		7
Other	11		2		13			5		4		9
Total noncurrent regulatory liabilities	$	2,664	$	1,629	$	4,387		$	1,215	$	768	$	1,985


(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’ electric distribution service annual revenue requirement calculated under the performance-based formula ratemaking framework and the revenue requirement included in customer rates for that year. Any under-recovery or over-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. Any 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 10 – Retirement Benefits for additional information.

(j)

The regulatory assets represent deferred income taxes that will be recovered from customers related to the equity component of allowance for funds used during construction and the effects of tax rate changes from the TCJA and the increased income tax rate in Illinois. The regulatory liabilities represent deferred income taxes that will be refunded to customers related to depreciation differences, other tax liabilities, and the unamortized portion of investment tax credits recorded at rates in excess of current statutory rates. Amounts associated with the equity component of allowance for funds used during construction, depreciation differences, and the unamortized portion of investment tax credits will be amortized over the expected life of the related assets. The amortization period for the effects of tax rate changes from the TCJA and the increased income tax rate in Illinois and the other tax liabilities will be determined in future rate orders by the applicable regulators. See Note 12 – Income Taxes for amounts related to the revaluation of deferred income taxes under the TCJA.


(k)	The tracker is amortized over three years, beginning from the date the amounts are included in rates. See Note 12 – 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 14 – 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. The MoPSC March 2017 electric rate order modified certain amortization periods for these costs. Costs incurred from May 2008 through September 2008, and from January 2010 through July 2012, are being amortized over a two-year period that began in April 2017. Costs incurred from October 2008 through December 2009 are no longer being amortized as of April 2017, and a new amortization period for these costs will be determined in a future regulatory rate review. Costs incurred from August 2012 through December 2012 are being amortized over a six-year period that began in June 2015.


(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 in 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. Costs incurred from 2010 to 2014 are being amortized over a two-year period that began in April 2017 as modified per the MoPSC March 2017 electric rate order. Costs incurred from 2015 to 2016 are being amortized over a three-year period that began in April 2017.

(v)

Electric energy-efficiency program investments deferred under the FEJA. These investments will earn a return at Ameren Illinois’ weighted-average cost of capital with the equity return based on the monthly average yield of the 30-year United States Treasury bonds plus 580 basis points. The investments are being amortized over their weighted-average useful lives beginning in the period in which they were made.


(w)	Estimated refunds to transmission customers related to the February 2015 FERC Complaint Case discussed above.


(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 2015 was refunded to customers from June 2016 through May 2017. The over-recovery relating to 2016 is being refunded to customers from June 2017 through May 2018. The over-recovery relating to 2017 will be refunded to customers from June 2018 through May 2019.

(z)

A regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri and the level of such costs included in customer rates. For costs incurred prior to August 2012, the amounts are being amortized over a two-year period that began in April 2017 as modified per the MoPSC’s March 2017 electric rate order. For costs incurred between August 2012 and December 2014, the MoPSC’s May 2015 electric rate order directed the amortization period to occur over a five-year period that began in June 2015. For costs incurred between January 2012 and December 2016, the MoPSC’s March 2017 electric rate order directed the amortization period to occur over a five-year period that began in April 2017. For costs incurred after December 2016, the amortization period will be determined in a future electric regulatory rate review.


(aa)	Funds collected from customers and alternative retail electric suppliers for the purchase of renewable energy credits and zero-emission credits through IPA procurements. The balance will be amortized as the 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 MoPSC’s March 2017 electric rate order directed the amortization to occur over a five-year period that began in April 2017. The April 2015 MoPSC order did not approve the continued use of the storm cost regulatory tracking mechanism.