Regulatory Matters (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Regulatory assets
Regulatory assets	$ 271,236	$ 271,344
Less current regulatory assets	(49,493)	(43,282)
Regulatory assets, non-current	221,743	228,062
Regulatory liabilities
Regulatory liabilities	527,599	536,652
Less current regulatory liabilities	(22,282)	(33,507)
Regulatory liabilities, non-current	505,317	503,145
Deferred energy and gas costs
Regulatory liabilities
Regulatory liabilities	14,443	17,278
Employee benefit plans
Regulatory liabilities
Regulatory liabilities	40,719	43,349
Cost of removal
Regulatory liabilities
Regulatory liabilities	169,426	166,727
Excess deferred income taxes
Regulatory liabilities
Regulatory liabilities	286,055	285,438
Other regulatory liabilities
Regulatory liabilities
Regulatory liabilities	16,956	23,860
Deferred energy and gas costs
Regulatory assets
Regulatory assets	35,878	34,088
Deferred gas cost adjustments
Regulatory assets
Regulatory assets	3,670	1,540
Gas price derivatives
Regulatory assets
Regulatory assets	499	3,328
Deferred taxes on AFUDC
Regulatory assets
Regulatory assets	7,683	7,790
Employee benefit plans
Regulatory assets
Regulatory assets	114,971	115,900
Environmental
Regulatory assets
Regulatory assets	1,417	1,454
Loss on reacquired debt
Regulatory assets
Regulatory assets	23,342	24,777
Renewable energy standard adjustment
Regulatory assets
Regulatory assets	0	1,622
Deferred taxes on flow through accounting
Regulatory assets
Regulatory assets	44,528	41,220
Decommissioning costs
Regulatory assets
Regulatory assets	9,421	10,670
Gas supply contract termination
Regulatory assets
Regulatory assets	4,027	8,485
Other regulatory assets
Regulatory assets
Regulatory assets	$ 25,800	$ 20,470