Revenue - Sources of Revenue (Details) $ in Thousands	12 Months Ended
Dec. 31, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 3,691,247
Retail residential electric service \| Retail residential
Disaggregation of Revenue [Line Items]
Total operating revenues	1,867,370
Retail residential electric service \| Retail non-residential
Disaggregation of Revenue [Line Items]
Total operating revenues	1,628,891
Retail residential electric service \| Wholesale
Disaggregation of Revenue [Line Items]
Total operating revenues	109,198
Transmission services for others
Disaggregation of Revenue [Line Items]
Total operating revenues	60,261
Other sources
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 25,527

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130543-203045

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130539-203045

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130545-203045

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration