Common Stock Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Class of Stock [Line Items]
Combined common and preferred stock, shares authorized	250,000,000
Common stock, shares authorized	200,000,000	200,000,000
Common stock, par or stated value per share	$ 0.01	$ 0.01
Preferred stock, shares authorized	50,000,000	50,000,000
Preferred stock, par or stated value per share	$ 0.01	$ 0.01
Common stock reserved for incentive plan awards	2,856,957
Net proceeds from sale of stock	$ 1,512	$ 74,904	$ 276,582
Shares paid for tax withholding	5,809	4,167
Proceeds from issuance of common stock, net	$ 0	$ 73,613	$ 276,971
Amount of Unrestricted Net Assets for Consolidated and Unconsolidated Subsidiaries	784,600
Amount of Unrestricted Net Assets for Northwestern Public Service Corporation Consolidated and Unconsolidated Subsidiaries	$ 294,600
Northwestern Corporation
Class of Stock [Line Items]
Common Equity Ratio Dividend Restriction Threshold	40.00%
Northwestern Public Service Corporation
Class of Stock [Line Items]
Common Equity Ratio Dividend Restriction Threshold	43.00%