Regulatory Matters (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Regulatory assets
Regulatory assets	$ 272,229	$ 271,344
Less current regulatory assets	(49,415)	(43,282)
Regulatory assets, non-current	222,814	228,062
Regulatory liabilities
Regulatory liabilities	558,494	536,652
Less current regulatory liabilities	(54,345)	(33,507)
Regulatory liabilities, non-current	504,149	503,145
Deferred energy and gas costs
Regulatory liabilities
Regulatory liabilities	40,002	17,278
Employee benefit plans
Regulatory liabilities
Regulatory liabilities	41,518	43,349
Cost of removal
Regulatory liabilities
Regulatory liabilities	170,954	166,727
Excess deferred income taxes
Regulatory liabilities
Regulatory liabilities	283,690	285,438
TCJA revenue reduction
Regulatory liabilities
Regulatory liabilities	5,147	3,418
Other regulatory liabilities
Regulatory liabilities
Regulatory liabilities	17,183	20,442
Deferred energy and gas costs
Regulatory assets
Regulatory assets	35,687	34,088
Deferred gas cost adjustments
Regulatory assets
Regulatory assets	0	1,540
Gas price derivatives
Regulatory assets
Regulatory assets	1,302	3,328
Deferred taxes on AFUDC
Regulatory assets
Regulatory assets	7,739	7,790
Employee benefit plans
Regulatory assets
Regulatory assets	117,150	115,900
Environmental
Regulatory assets
Regulatory assets	1,439	1,454
Loss on reacquired debt
Regulatory assets
Regulatory assets	24,299	24,777
Renewable energy standard adjustment
Regulatory assets
Regulatory assets	340	1,622
Deferred taxes on flow through accounting
Regulatory assets
Regulatory assets	44,589	41,220
Decommissioning costs
Regulatory assets
Regulatory assets	10,248	10,670
Gas supply contract termination
Regulatory assets
Regulatory assets	7,007	8,485
Other regulatory assets
Regulatory assets
Regulatory assets	$ 22,429	$ 20,470