Subordinated Debentures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Terms of Subordinated Debentures
Balance	$ 139,959	$ 139,914
Subordinated debentures owed to trust subsidiaries
Terms of Subordinated Debentures
Balance	132,459
Tier 2 subordinated debentures
Terms of Subordinated Debentures
Balance	$ 7,500
Rate at December 31, 2020	6.625%
First Company Statutory Trust 2001 | Subordinated debentures owed to trust subsidiaries
Terms of Subordinated Debentures
Balance	$ 3,458
Rate at December 31, 2020	3.514%
LIBOR rate spread	3.30%
First Company Statutory Trust 2003 | Subordinated debentures owed to trust subsidiaries
Terms of Subordinated Debentures
Balance	$ 2,542
Rate at December 31, 2020	3.501%
LIBOR rate spread	3.25%
Glacier Capital Trust II | Subordinated debentures owed to trust subsidiaries
Terms of Subordinated Debentures
Balance	$ 46,393
Rate at December 31, 2020	2.987%
LIBOR rate spread	2.75%
Citizens (ID) Statutory Trust I | Subordinated debentures owed to trust subsidiaries
Terms of Subordinated Debentures
Balance	$ 5,155
Rate at December 31, 2020	2.879%
LIBOR rate spread	2.65%
Glacier Capital Trust III | Subordinated debentures owed to trust subsidiaries
Terms of Subordinated Debentures
Balance	$ 36,083
Rate at December 31, 2020	1.527%
LIBOR rate spread	1.29%
Glacier Capital Trust IV | Subordinated debentures owed to trust subsidiaries
Terms of Subordinated Debentures
Balance	$ 30,928
Rate at December 31, 2020	1.787%
LIBOR rate spread	1.57%
Bank of the San Juans Bancorporation Trust I | Subordinated debentures owed to trust subsidiaries
Terms of Subordinated Debentures
Balance	$ 2,006
Rate at December 31, 2020	2.045%
LIBOR rate spread	1.82%
FNB (UT) Statutory Trust I | Subordinated debentures owed to trust subsidiaries
Terms of Subordinated Debentures
Balance	$ 4,124
Rate at December 31, 2020	3.351%
LIBOR rate spread	3.10%
FNB (UT) Statutory Trust II | Subordinated debentures owed to trust subsidiaries
Terms of Subordinated Debentures
Balance	$ 1,770
Rate at December 31, 2020	1.937%
LIBOR rate spread	1.72%