3. Revenues

Our revenues are derived primarily from the sale of natural gas and electricity to retail customers. We also record revenue from sales of natural gas and electricity to wholesale counterparties, including affiliates. Revenue is measured based upon the quantity of gas or power delivered to the retail customer or wholesale counterparty at prices contained or referenced in the customer's contract, and excludes any sales incentives (e.g. rebates) and amounts collected on behalf of third parties (e.g. sales tax).

We record gross receipts taxes on a gross basis in retail revenues and retail cost of revenues. During the three months ended September 30, 2018 and 2017, our retail revenues included gross receipts taxes of $2.2 million and $2.1 million, respectively. During the three months ended September 30, 2018 and 2017, our retail cost of revenues included gross receipts taxes of $2.7 million and $2.7 million, respectively. During the nine months ended September 30, 2018 and 2017, our retail revenues included gross receipts taxes of $6.5 million and $4.6 million, respectively. During the nine months ended September 30, 2018 and 2017, our retail cost of revenues included gross receipts taxes of $7.8 million and $6.6 million, respectively.

Our revenues also include asset optimization activities. Asset optimization activities consist primarily of purchases and sales of gas that meet the definition of trading activities per FASB ASC Topic 815, Derivatives and Hedging. They are therefore excluded from the scope of Revenue from Contracts with Customers (Topic 606).

The following is a description of our principal revenue generating activities.

Retail Electricity

Revenues for electricity sales are recognized under the accrual method when our performance obligation to a customer is satisfied, which is the point in time when the product is delivered and control of the product passes to the customer. Electricity products may be sold as fixed or variable rate products. The typical length of a contract to provide electricity is 12 months. Customers are billed and typically pay at least monthly, based on usage. Electricity sales that have been delivered but not billed by period end are estimated. Accrued unbilled revenues are based on estimates of customer usage since the date of the last meter read provided by the utility. Volume estimates are based on forecasted volumes and estimated residential and commercial customer usage. Unbilled revenues are calculated by multiplying these volume estimates by the applicable rate by customer class (residential or commercial). Estimated amounts are adjusted when actual usage is known and billed.

Retail Natural Gas

Revenues for natural gas sales are recognized under the accrual method when our performance obligation to a customer is satisfied, which is the point in time when the product is delivered and control of the product passes to the customer. Natural gas products may be sold as fixed-price or variable-price products. The typical length of a contract to provide natural gas is 12 months. Customers are billed and typically pay at least monthly, based on usage. Natural gas sales that have been delivered but not billed by period end are estimated and recorded as accrued unbilled revenues based on estimates of customer usage since the date of the last meter read provided by the utility. Volume estimates are based on forecasted volumes and estimated residential and commercial customer usage. Unbilled revenues are calculated by multiplying these volume estimates by the applicable rate by customer class (residential or commercial). Estimated amounts are adjusted when actual usage is known and billed.

The following table discloses revenue by primary geographical market, customer type, and customer credit risk profile (in thousands). The table also includes a reconciliation of the disaggregated revenue to revenue by reportable segment (in thousands).



	Reportable Segments
	Three Months Ended September 30, 2018								Three Months Ended September 30, 2017
	Retail Electricity			Retail Natural Gas		Total Reportable Segments			Retail Electricity		Retail Natural Gas		Total Reportable Segments


Primary markets (a)
New England	$	110,870		$	2,163	$	113,033		$	61,421	$	2,157	$	63,578
Mid-Atlantic	83,846			3,762		87,608			83,955		4,543		88,498
Midwest	20,898			2,557		23,455			20,111		2,570		22,681
Southwest	30,568			3,463		34,031			36,772		4,327		41,099
	$	246,182		$	11,945	$	258,127		$	202,259	$	13,597	$	215,856

Customer type
Commercial	$	101,818		$	4,650	$	106,468		$	55,489	$	5,004	$	60,493
Residential	151,918			7,068		158,986			143,152		8,571		151,723
Unbilled revenue (b)	(7,554		)	227		(7,327		)	3,618		22		3,640
	$	246,182		$	11,945	$	258,127		$	202,259	$	13,597	$	215,856

Customer credit risk
POR	$	172,198		$	5,013	$	177,211		$	138,544	$	5,963	$	144,507
Non-POR	73,984			6,932		80,916			63,715		7,634		71,349
	$	246,182		$	11,945	$	258,127		$	202,259	$	13,597	$	215,856



	Reportable Segments
	Nine Months Ended September 30, 2018									Nine Months Ended September 30, 2017
	Retail Electricity			Retail Natural Gas			Total Reportable Segments			Retail Electricity		Retail Natural Gas			Total Reportable Segments


Primary markets (a)
New England	$	305,894		$	14,742		$	320,636		$	157,334	$	15,252		$	172,586
Mid-Atlantic	229,329			39,112			268,441			197,877		35,664			233,541
Midwest	56,818			27,243			84,061			43,073		23,893			66,966
Southwest	84,487			15,991			100,478			69,577		21,290			90,867
	$	676,528		$	97,088		$	773,616		$	467,861	$	96,099		$	563,960

Customer type
Commercial	$	275,966		$	39,826		$	315,792		$	140,408	$	40,224		$	180,632
Residential	415,022			73,138			488,160			322,354		70,886			393,240
Unbilled revenue (b)	(14,460		)	(15,876		)	(30,336		)	5,099		(15,011		)	(9,912		)
	$	676,528		$	97,088		$	773,616		$	467,861	$	96,099		$	563,960

Customer credit risk
POR	$	473,438		$	54,565		$	528,003		$	318,440	$	47,907		$	366,347
Non-POR	203,090			42,523			245,613			149,421		48,192			197,613
	$	676,528		$	97,088		$	773,616		$	467,861	$	96,099		$	563,960

(a) The primary markets noted above include the following states:


•	New England - Connecticut, Maine, Massachusetts, New Hampshire;


•	Mid-Atlantic - Delaware, Maryland (including the District of Colombia), New Jersey, New York and Pennsylvania;


•	Midwest - Illinois, Indiana, Michigan and Ohio; and


•	Southwest - Arizona, California, Colorado, Florida, Nevada, and Texas.

(b) Unbilled revenue is recorded in total until it is actualized, at which time it is categorized between commercial and residential customers.