Subordinated Debentures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Terms of Subordinated Debentures
Balance	$ 139,914	$ 134,051
Subordinated debentures owed to trust subsidiaries
Terms of Subordinated Debentures
Balance	132,297
Tier 2 subordinated debentures
Terms of Subordinated Debentures
Balance	$ 7,617
Rate at December 31, 2019	6.625%
First Company Statutory Trust 2001 | Subordinated debentures owed to trust subsidiaries
Terms of Subordinated Debentures
Balance	$ 3,395
Rate at December 31, 2019	5.227%
LIBOR Rate Spread	3.30%
First Company Statutory Trust 2003 | Subordinated debentures owed to trust subsidiaries
Terms of Subordinated Debentures
Balance	$ 2,497
Rate at December 31, 2019	5.197%
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, 2019	4.736%
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, 2019	4.55%
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, 2019	3.276%
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, 2019	3.464%
LIBOR Rate Spread	1.57%
Bank of the San Juans Bancorporation Trust I | Subordinated debentures owed to trust subsidiaries
Terms of Subordinated Debentures
Balance	$ 1,970
Rate at December 31, 2019	3.734%
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, 2019	5.061%
LIBOR Rate Spread	3.10%
FNB (UT) Statutory Trust II | Subordinated debentures owed to trust subsidiaries
Terms of Subordinated Debentures
Balance	$ 1,752
Rate at December 31, 2019	3.614%
LIBOR Rate Spread	1.72%