NRG's goodwill balance was $579 million as of December 31, 2020 and 2019. As of December 31, 2020, goodwill consisted of $165 million associated with the acquisition of Midwest Generation and $414 million for retail operations acquisitions, including Stream Energy and XOOM, which were acquired in 2019 and 2018, respectively.

The Company's intangible assets as of December 31, 2020, primarily reflect intangible assets established with the acquisitions of various companies, including Stream Energy, XOOM, other retail acquisitions, and Texas Genco. Intangible assets are comprised of the following:

•Emission Allowances — These intangibles primarily consist of SO2 emission allowances, including those established with the 2006 acquisition of Texas Genco, RGGI emission credits and California carbon allowances. These emission allowances are held-for-use and are amortized to cost of operations based on units of production.

•In-market nuclear fuel contracts — These intangibles were established with the Texas Genco acquisition in 2006 and are amortized to cost of operations over expected volumes over the life of each contract.

•Customer relationships — These intangibles represent the fair value at the acquisition date of acquired businesses' customer base. The customer relationships are amortized to depreciation and amortization expense based on the expected discounted future net cash flows by year.

•Marketing partnerships — These intangibles represent the fair value at the acquisition date of existing agreements with marketing vendors and loyalty and affinity partners for customer acquisition. The marketing partnerships are amortized to depreciation and amortization expense based on the expected discounted future net cash flows by year.

•Other — Consists of renewable energy credits, costs to extend the operating license for STP Units 1 and 2, and energy supply contracts acquired with Stream Energy that represent the fair value at the acquisition date of in-market contracts for the purchase of energy to serve retail electric customers. Renewable energy credits are retired, as required, for the applicable compliance period. They are expensed to cost of operations based on NRG’s customer usage. Energy supply contracts are amortized to depreciation and amortization based on the expected delivery under the respective contracts.


(In millions)
Year Ended December 31, 2020	Emission Allowances		Fuel Contracts		Customer Relationships		Marketing Partnerships		Trade Names		Other(b)		Total
January 1, 2020	$	662	$	49	$	573	$	285	$	373	$	109	$	2,051
Purchases	25		—		—		—		—		45		70
Acquisition of businesses (a)	—		—		22		—		—		—		22
Retirements	—		—		—		—		—		(35)		(35)
Write-off of fully amortized balances	(4)		—		(70)		—		—		—		(74)
Impairment	(14)		—		—		—		—		—		(14)
Other	3		—		2		—		—		—		5
December 31, 2020	672		49		527		285		373		119		2,025
Less accumulated amortization	(563)		(46)		(349)		(99)		(247)		(53)		(1,357)
Net carrying amount	$	109	$	3	$	178	$	186	$	126	$	66	$	668


(In millions)
Year Ended December 31, 2019	Emission Allowances		Fuel Contracts		Customer Relationships		Marketing Partnerships		Trade Names		Other(b)		Total
January 1, 2019	$	659	$	49	$	478	$	131	$	345	$	80	$	1,742
Purchases	13		—		—		—		—		29		42
Acquisition of businesses (a)	—		—		110		154		28		26		318
Usage/Retirements	(4)		—		—		—		—		(17)		(21)
Write-off of fully amortized balances	(8)		—		(13)		—		—		(9)		(30)
Impairment	—		—		(2)		—		—		—		(2)
Other	2		—		—		—		—		—		2
December 31, 2019	662		49		573		285		373		109		2,051
Less accumulated amortization	(539)		(45)		(345)		(75)		(220)		(38)		(1,262)
Net carrying amount	$	123	$	4	$	228	$	210	$	153	$	71	$	789

(a)The weighted average life of acquired intangibles was: customer relationships 7 years, trade names 12 years, marketing partnerships 9 years, and energy supply contracts 2 years


	Years Ended December 31,
(In millions)	2020		2019		2018
Emission allowances	$	28	$	32	$	39
Customer relationships	74		44		32
Marketing partnerships	24		15		9
Trade names	27		25		23
Other(a)	51		35		30
Total amortization	$	204	$	151	$	133

(a)For the years ended December 31, 2020, 2019 and 2018, RECs were retired to cost of operations for $36 million, $17 million and $28 million, respectively. For the years ended December 31, 2020, 2019 and 2018, other intangibles were amortized to depreciation and amortization expense for $15 million, $18 million and $2 million, respectively.

The following table presents estimated amortization of NRG's intangible assets as of December 31, 2020 for each of the next five years:


(In millions)
Year Ended December 31,	Emission Allowances		Fuel Contracts		Customer Relationships		Marketing Partnerships		Trade Names		Other		Total
2021	$	31	$	—	$	64	$	24	$	27	$	3	$	149
2022	30		1		44		23		27		3		128
2023	30		—		45		23		26		3		127
2024	31		1		16		23		17		3		91
2025	29		—		5		22		11		3		70

Intangible assets held-for-sale — From time to time, management may authorize the transfer from the Company's emission bank of emission allowances held-for-use to intangible assets held-for-sale. Emission allowances held-for-sale are included in other non-current assets on the Company's consolidated balance sheet and are not amortized, but rather expensed as sold. As of December 31, 2020 and 2019, the value of emission allowances held-for-sale was $14 million and $6 million, respectively, within the Corporate segment. Once transferred to held-for-sale, these emission allowances are prohibited from moving back to held-for-use.