Income Taxes - Summary of Significant Factors Impact on Difference Between Effective Tax Rate and Federal Statutory Rate (Parenthetical) (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Effective Tax Rate Reconciliation [Line Items]
Regulatory liabilities			$ 49,629		$ 49,629		$ 51,715
Non-plant excess deferred turnaround, amount			(8,390)	$ (5,091)	(8,476)	$ (5,601)
WUTC
Effective Tax Rate Reconciliation [Line Items]
Non-plant excess deferred turnaround, amount	$ 8,400
IPUC
Effective Tax Rate Reconciliation [Line Items]
Non-plant excess deferred turnaround, amount		$ 5,100
Income Tax Related
Effective Tax Rate Reconciliation [Line Items]
Regulatory liabilities			$ 13,335		$ 13,335		$ 23,803
Income Tax Related | WUTC
Effective Tax Rate Reconciliation [Line Items]
Regulatory liabilities	$ 10,900
Income Tax Related | IPUC
Effective Tax Rate Reconciliation [Line Items]
Regulatory liabilities		$ 6,400