Subordinated Debentures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Terms of Subordinated Debentures
Balance	$ 134,051	$ 126,135
Subordinated debentures owed to trust subsidiaries
Terms of Subordinated Debentures
Balance	126,278
Tier 2 subordinated debentures
Terms of Subordinated Debentures
Balance	$ 7,773
Rate at December 31, 2018	6.625%
First Company Statutory Trust 2001 | Subordinated debentures owed to trust subsidiaries
Terms of Subordinated Debentures
Balance	$ 3,332
Rate at December 31, 2018	5.827%
LIBOR Rate Spread	3.30%
First Company Statutory Trust 2003 | Subordinated debentures owed to trust subsidiaries
Terms of Subordinated Debentures
Balance	$ 2,452
Rate at December 31, 2018	6.072%
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, 2018	5.186%
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, 2018	5.438%
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, 2018	3.726%
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, 2018	4.358%
LIBOR Rate Spread	1.57%
Bank of the San Juans Bancorporation Trust I | Subordinated debentures owed to trust subsidiaries
Terms of Subordinated Debentures
Balance	$ 1,935
Rate at December 31, 2018	4.558%
LIBOR Rate Spread	1.82%