Business Combinations - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended
	Feb. 04, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Business Acquisition [Line Items]
Electric operations revenue		$ 386,388	$ 451,902		$ 1,019,501	$ 1,130,318
Other operations revenue		(47,597)	(49,780)		(137,407)	(131,544)
Depreciation expense		$ 52,892	57,004		$ 162,852	158,395
South Central Generating
Business Acquisition [Line Items]
Consideration transferred	$ 962,200
Acquisition price	1,000,000
Working capital and other adjustments	$ 37,800
Pro forma net income			56,004			122,771
South Central Generating | Fair Value Adjustments
Business Acquisition [Line Items]
Electric operations revenue				$ 500
Other operations revenue				100
Depreciation expense				$ 200
South Central Generating | Transaction-related Expenses
Business Acquisition [Line Items]
Pro forma net income			$ 600			$ 4,500