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Revenue:
 12 Months Ended
Dec. 31, 2020
Revenue from Contract with Customer [Abstract]  
Revenue REVENUE
Our revenue contracts generally provide for performance obligations that are fulfilled and transfer control to customers over time, represent a series of distinct services that are substantially the same, involve the same pattern of transfer to the customer and provide a right to consideration from our customers in an amount that corresponds directly with the value to the customer for the performance completed to date. Therefore, we recognize revenue in the amount to which we have a right to invoice. Our primary types of revenue contracts are:

•Regulated natural gas and electric utility services tariffs - Our utilities have regulated operations, as defined by ASC 980, Regulated Operations, that provide services to regulated customers under tariff rates, charges, terms and conditions of service and prices determined by the jurisdictional regulators designated for our service territories. Our regulated services primarily encompass single performance obligations for delivery of either commodity natural gas, commodity electricity, natural gas transportation or electric transmission services. These service revenues are variable based on quantities delivered, influenced by seasonal business and weather patterns. Tariffs are only permitted to be changed through a rate-setting process involving the state or federal regulatory commissions to establish contractual rates between the utility and its customers. All of our Utilities’ regulated sales are subject to regulatory-approved tariffs.

•Power sales agreements - Our Electric Utilities and Power Generation segments have long-term wholesale power sales agreements with other load-serving entities, including affiliates, for the sale of excess power from owned generating units. These agreements include a combination of “take or pay” arrangements, where the customer is obligated to pay for the energy regardless of whether it actually takes delivery, as well as “requirements only” arrangements, where the customer is only obligated to pay for the energy the customer needs. In addition to these long-term contracts, we also sell excess energy to other load-serving entities on a short-term basis. The pricing for all of these arrangements is included in the executed contracts or confirmations, reflecting the standalone selling price and is variable based on energy delivered. Certain energy sale and purchase transactions with the same counterparty and at the same delivery point are netted to reflect the economic substance of the arrangement.

•Coal supply agreements - Our Mining segment sells coal primarily under long-term contracts to utilities for use at their power generating plants, including affiliate Electric Utilities, and an affiliate non-regulated Power Generation entity. The contracts include a single promise to supply coal necessary to fuel the customers’ facilities during the contract term. The transaction price is established in the supply agreements, including cost-based agreements with the affiliated regulated utilities, and is variable based on tons delivered.

•Other non-regulated services - Our Electric and Gas Utilities segments also provide non-regulated services primarily comprised of appliance repair service and protection plans, electric and natural gas technical infrastructure construction and maintenance services, and in Nebraska and Wyoming, an unbundled natural gas commodity offering under the regulatory-approved Choice Gas Program. Revenue contracts for these services generally represent a single performance obligation with the price reflecting the standalone selling price stated in the agreement, and the revenue is variable based on the units delivered or services provided.

The following tables depict the disaggregation of revenue, including intercompany revenue, from contracts with customers by customer type and timing of revenue recognition for each of the reportable segments, for the years ended December 31, 2020, 2019 and 2018. Sales tax and other similar taxes are excluded from revenues.
Year ended December 31, 2020 Electric Utilities  Gas Utilities Power Generation MiningInter-company RevenuesTotal
Customer types:(in thousands)
Retail$610,721 $765,922 $— $58,567 $(31,478)$1,403,732 
Transportation— 154,581 — — (526)154,055 
Wholesale17,848 — 103,258 — (97,169)23,937 
Market - off-system sales24,309 260 — — (8,797)15,772 
Transmission/Other58,965 43,658 — — (19,315)83,308 
Revenue from contracts with customers711,843 964,421 103,258 58,567 (157,285)1,680,804 
Other revenues2,201 10,249 1,789 2,508 (610)16,137 
Total revenues$714,044 $974,670 $105,047 $61,075 $(157,895)$1,696,941 
Timing of revenue recognition:
Services transferred at a point in time$— $— $— $58,567 $(31,478)$27,089 
Services transferred over time711,843 964,421 103,258 — (125,807)1,653,715 
Revenue from contracts with customers$711,843 $964,421 $103,258 $58,567 $(157,285)$1,680,804 
Year ended December 31, 2019 Electric Utilities  Gas Utilities Power Generation MiningInter-company RevenuesTotal
Customer types:(in thousands)
Retail$605,756 $817,840 $— $59,233 $(32,053)$1,450,776 
Transportation— 143,390 — — (1,042)142,348 
Wholesale20,884 — 99,157 — (91,577)28,464 
Market - off-system sales23,817 691 — — (7,736)16,772 
Transmission/Other57,104 47,725 — — (16,797)88,032 
Revenue from contracts with customers707,561 1,009,646 99,157 59,233 (149,205)1,726,392 
Other revenues5,191 384 2,101 2,396 (1,564)8,508 
Total revenues$712,752 $1,010,030 $101,258 $61,629 $(150,769)$1,734,900 
Timing of revenue recognition:
Services transferred at a point in time$— $— $— $59,233 $(32,053)$27,180 
Services transferred over time707,561 1,009,646 99,157 — (117,152)1,699,212 
Revenue from contracts with customers$707,561 $1,009,646 $99,157 $59,233 $(149,205)$1,726,392 
Year ended December 31, 2018 Electric Utilities  Gas Utilities Power Generation MiningInter-company RevenuesTotal
Customer types:(in thousands)
Retail$594,329 $833,379 $— $65,803 $(32,194)$1,461,317 
Transportation— 140,705 — — (1,348)139,357 
Wholesale33,687 — 90,791 — (84,957)39,521 
Market - off-system sales24,799 866 — — (8,102)17,563 
Transmission/Other56,209 49,402 — — (14,827)90,784 
Revenue from contracts with customers709,024 1,024,352 90,791 65,803 (141,428)1,748,542 
Other revenues2,427 955 1,660 2,230 (1,546)5,726 
Total revenues$711,451 $1,025,307 $92,451 $68,033 $(142,974)$1,754,268 
Timing of revenue recognition:
Services transferred at a point in time$— $— $— $65,803 $(32,194)$33,609 
Services transferred over time709,024 1,024,352 90,791 — (109,234)1,714,933 
Revenue from contracts with customers$709,024 $1,024,352 $90,791 $65,803 $(141,428)$1,748,542 

The majority of our revenue contracts are based on variable quantities delivered. Any fixed consideration contracts with an expected duration of one year or more are immaterial to our consolidated revenues. Variable consideration constraints in the form of discounts, rebates, credits, price concessions, incentives, performance bonuses, penalties or other similar items are not material for our revenue contracts. We are the principal in our revenue contracts, as we have control over the services prior to those services being transferred to the customer.

Revenue Not in Scope of ASC 606
Other revenues included in the table above include our revenue accounted for under separate accounting guidance, including lease revenue under ASC 842, Leases, derivative revenue under ASC 815, Derivatives and Hedging, and alternative revenue programs revenue under ASC 980, Regulated Operations.

Significant Judgments and Estimates
Unbilled Revenue

To the extent that deliveries have occurred but a bill has not been issued, our utilities accrue an estimate of the revenue since the latest billing. This estimate is calculated based upon several factors including billings through the last billing cycle in a month and prices in effect in our jurisdictions. Each month, the estimated unbilled revenue amounts are trued-up and recorded in Accounts receivable, net on the accompanying Consolidated Balance Sheets.

Contract Balances

The nature of our primary revenue contracts provides an unconditional right to consideration upon service delivery; therefore, no customer contract assets or liabilities exist. The unconditional right to consideration is represented by the balance in our Accounts Receivable further discussed in Note 1.