Pension Plans and Other Postretirement Benefit Plans - Additional Information (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Payment for pension benefits	$ 14,600	$ 14,600
Percentage of service related net periodic benefit costs capitalized to utility property	40.00%
Percentage of service related net periodic benefit costs recorded to operating expenses	60.00%
Percentage of non-service related net periodic benefit costs capitalized to regulatory assets	40.00%
Percentage of non-service related net periodic benefit costs recorded to other expense	60.00%
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected future employer contributions, next fiscal year	$ 22,000