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Regulatory Accounting (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Regulatory assets    
Regulatory assets $ 283,049 $ 284,235
Less current regulatory assets (48,925) (48,776)
Regulatory assets, non-current 234,124 235,459
Regulatory liabilities    
Regulatory liabilities 554,556 540,794
Less current regulatory liabilities (39,642) (29,810)
Regulatory liabilities, non-current 514,914 510,984
Deferred energy and gas costs    
Regulatory liabilities    
Regulatory liabilities 16,808 6,991
Employee benefit plans    
Regulatory liabilities    
Regulatory liabilities 41,814 42,533
Cost of removal    
Regulatory liabilities    
Regulatory liabilities 158,477 150,123
Excess deferred income taxes    
Regulatory liabilities    
Regulatory liabilities 307,871 310,562
TCJA revenue reduction    
Regulatory liabilities    
Regulatory liabilities 11,436 18,032
Other regulatory liabilities    
Regulatory liabilities    
Regulatory liabilities 18,150 12,553
Deferred energy and gas costs    
Regulatory assets    
Regulatory assets 34,257 29,661
Deferred gas cost adjustments    
Regulatory assets    
Regulatory assets 2,342 3,362
Gas price derivatives    
Regulatory assets    
Regulatory assets 3,945 6,201
Deferred taxes on AFUDC    
Regulatory assets    
Regulatory assets 7,716 7,841
Employee benefit plans    
Regulatory assets    
Regulatory assets 109,899 110,524
Environmental    
Regulatory assets    
Regulatory assets 931 959
Loss on reacquired debt    
Regulatory assets    
Regulatory assets 20,140 21,001
Renewable energy standard adjustment    
Regulatory assets    
Regulatory assets 1,994 1,722
Deferred taxes on flow through accounting    
Regulatory assets    
Regulatory assets 36,552 31,044
Decommissioning costs    
Regulatory assets    
Regulatory assets 11,518 11,700
Gas supply contract termination    
Regulatory assets    
Regulatory assets 11,413 14,310
Other regulatory assets    
Regulatory assets    
Regulatory assets $ 42,342 $ 45,910