Summary of Significant Accounting Policies - Other Expense (Income) - Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Interest income	$ (1,952)	$ (2,587)	$ (2,710)
Interest on regulatory deferrals	(1,222)	(1,460)	(990)
Equity-related AFUDC	(6,970)	(6,585)	(6,554)
Non-service portion of pension and other postretirement benefit expenses	6,433	8,899	5,156
Net (income) loss on investments	(905)	(14,299)	5,369
Other expense (income)	(201)	1,104	1,187
Total	$ (4,817)	$ (14,928)	$ 1,458