Operating Segment Information (Details) (USD $) In Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Financial data for Company's operating segments
External revenues	$ 89,413	$ 95,236	$ 175,181	$ 184,817
Expenses	(77,527)	(82,014)	(153,010)	(161,525)
Net Earnings	11,886	13,222	22,171	23,292	42,330
Total assets	6,978,320	6,294,836	6,978,320	6,294,836	6,759,287
Glacier [Member]
Financial data for Company's operating segments
External revenues	17,936	18,969	35,270	37,704
Intersegment revenues	74	48	140	96
Expenses	(16,198)	(16,407)	(31,239)	(34,142)
Net Earnings	1,812	2,610	4,171	3,658
Total assets	1,379,298	1,320,555	1,379,298	1,320,555
Mountain West [Member]
Financial data for Company's operating segments
External revenues	17,682	22,183	35,151	41,133
Intersegment revenues	97	19	239	38
Expenses	(19,764)	(21,759)	(38,273)	(40,243)
Net Earnings	(1,985)	443	(2,883)	928
Total assets	1,152,583	1,200,382	1,152,583	1,200,382
First Security [Member]
Financial data for Company's operating segments
External revenues	13,510	13,097	26,167	25,653
Intersegment revenues	19	20	39	38
Expenses	(10,059)	(10,057)	(20,773)	(20,217)
Net Earnings	3,470	3,060	5,433	5,474
Total assets	1,082,737	932,179	1,082,737	932,179
Western [Member]
Financial data for Company's operating segments
External revenues	8,851	8,811	17,045	16,939
Intersegment revenues	24	123	79	255
Expenses	(6,356)	(6,686)	(12,661)	(13,003)
Net Earnings	2,519	2,248	4,463	4,191
Total assets	778,081	610,208	778,081	610,208
1st Bank [Member]
Financial data for Company's operating segments
External revenues	8,089	7,753	15,702	15,729
Intersegment revenues	9	30	12	121
Expenses	(6,119)	(6,919)	(12,634)	(13,420)
Net Earnings	1,979	864	3,080	2,430
Total assets	756,704	644,877	756,704	644,877
Valley [Member]
Financial data for Company's operating segments
External revenues	5,471	5,798	10,440	10,890
Intersegment revenues	67	40	126	76
Expenses	(3,751)	(3,921)	(7,244)	(7,552)
Net Earnings	1,787	1,917	3,322	3,414
Total assets	411,619	368,321	411,619	368,321
Big Sky [Member]
Financial data for Company's operating segments
External revenues	4,753	5,099	9,494	9,935
Intersegment revenues	4	1	7	1
Expenses	(3,740)	(4,397)	(7,522)	(8,901)
Net Earnings	1,017	703	1,979	1,035
Total assets	373,530	366,439	373,530	366,439
First Bank-WY [Member]
Financial data for Company's operating segments
External revenues	3,850	3,659	7,111	7,699
Intersegment revenues	29	14	64	22
Expenses	(2,980)	(3,180)	(5,733)	(6,856)
Net Earnings	899	493	1,442	865
Total assets	373,014	295,164	373,014	295,164
Citizens [Member]
Financial data for Company's operating segments
External revenues	3,704	4,608	7,860	8,756
Intersegment revenues	12	28	30	28
Expenses	(2,980)	(3,842)	(6,563)	(7,412)
Net Earnings	736	794	1,327	1,372
Total assets	317,280	271,190	317,280	271,190
First Bank-MT [Member]
Financial data for Company's operating segments
External revenues	2,544	2,472	4,858	4,892
Intersegment revenues	39	32	74	82
Expenses	(1,639)	(1,705)	(3,120)	(3,396)
Net Earnings	944	799	1,812	1,578
Total assets	243,141	193,806	243,141	193,806
San Juans [Member]
Financial data for Company's operating segments
External revenues	2,610	2,688	5,239	5,325
Intersegment revenues	42	24	86	24
Expenses	(2,393)	(2,135)	(4,603)	(4,596)
Net Earnings	259	577	722	753
Total assets	226,032	204,815	226,032	204,815
GORE [Member]
Financial data for Company's operating segments
External revenues	219	43	249	43
Expenses	(801)	(268)	(1,125)	(268)
Net Earnings	(582)	(225)	(876)	(225)
Total assets	20,318	19,856	20,318	19,856
Parent [Member]
Financial data for Company's operating segments
External revenues	194	56	595	119
Intersegment revenues	16,473	17,885	31,159	32,521
Expenses	(4,806)	(4,719)	(9,668)	(9,348)
Net Earnings	11,861	13,222	22,086	23,292
Total assets	1,009,907	985,895	1,009,907	985,895
Eliminations [Member]
Financial data for Company's operating segments
Intersegment revenues	(16,889)	(18,264)	(32,055)	(33,302)
Expenses	4,059	3,981	8,148	7,829
Net Earnings	(12,830)	(14,283)	(23,907)	(25,473)
Total assets	$ (1,145,924)	$ (1,118,851)	$ (1,145,924)	$ (1,118,851)