Regulatory Matters (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Regulatory assets
Regulatory assets	$ 272,312	$ 271,344
Less current regulatory assets	(51,745)	(43,282)
Regulatory assets, non-current	220,567	228,062
Regulatory liabilities
Regulatory liabilities	565,821	536,652
Less current regulatory liabilities	(59,428)	(33,507)
Regulatory liabilities, non-current	506,393	503,145
Deferred energy and gas costs
Regulatory liabilities
Regulatory liabilities	28,279	17,278
Employee benefit plans
Regulatory liabilities
Regulatory liabilities	41,025	43,349
Cost of removal
Regulatory liabilities
Regulatory liabilities	168,137	166,727
Excess deferred income taxes
Regulatory liabilities
Regulatory liabilities	288,796	285,438
TCJA revenue reduction
Regulatory liabilities
Regulatory liabilities	3,027	3,418
Other regulatory liabilities
Regulatory liabilities
Regulatory liabilities	36,557	20,442
Deferred energy and gas costs
Regulatory assets
Regulatory assets	36,900	34,088
Deferred gas cost adjustments
Regulatory assets
Regulatory assets	226	1,540
Gas price derivatives
Regulatory assets
Regulatory assets	662	3,328
Deferred taxes on AFUDC
Regulatory assets
Regulatory assets	7,715	7,790
Employee benefit plans
Regulatory assets
Regulatory assets	115,416	115,900
Environmental
Regulatory assets
Regulatory assets	1,426	1,454
Loss on reacquired debt
Regulatory assets
Regulatory assets	23,820	24,777
Renewable energy standard adjustment
Regulatory assets
Regulatory assets	1	1,622
Deferred taxes on flow through accounting
Regulatory assets
Regulatory assets	44,940	41,220
Decommissioning costs
Regulatory assets
Regulatory assets	9,854	10,670
Gas supply contract termination
Regulatory assets
Regulatory assets	5,521	8,485
Other regulatory assets
Regulatory assets
Regulatory assets	$ 25,831	$ 20,470