Employee Benefit Plans Investment Strategy (Details)	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	5.00%
South Dakota | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	100.00%	100.00%
South Dakota | Cash and cash equivalents | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.90%	0.00%
South Dakota | Domestic debt securities | Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	80.00%	75.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	77.00%	81.30%
South Dakota | International debt securities | Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	2.00%	2.50%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	2.60%	2.60%
South Dakota | Domestic equity securities | Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	7.20%	9.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	8.10%	6.30%
South Dakota | International equity securities | Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	10.80%	13.50%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	11.40%	9.80%
Montana | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	100.00%	100.00%
Montana | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	100.00%	100.00%
Montana | Cash and cash equivalents | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.00%	0.10%
Montana | Cash and cash equivalents | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	1.00%	1.00%
Montana | Domestic debt securities | Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	55.00%	55.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	53.80%	57.50%
Montana | Domestic debt securities | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	40.00%	40.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	37.80%	40.80%
Montana | International debt securities | Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	4.00%	4.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	4.00%	4.40%
Montana | International debt securities | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	0.00%	0.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.00%	0.00%
Montana | Domestic equity securities | Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	16.50%	16.50%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	16.80%	15.00%
Montana | Domestic equity securities | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	50.00%	50.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	52.40%	49.10%
Montana | International equity securities | Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	24.50%	24.50%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	25.40%	23.00%
Montana | International equity securities | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	10.00%	10.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	8.80%	9.10%