Investment Securities - Realized Gains (Losses) on Investments (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Investments, Debt and Equity Securities [Abstract]
Gross realized gains	$ 351	$ 1,544	$ 173
Gross realized losses	3	0	3
Political Subdivisions
Schedule of Available-for-sale Securities
Available-for-sale Securities, Amortized Cost Basis	139,434
Montana, Wyoming, and South Dakota \| Political Subdivisions
Schedule of Available-for-sale Securities
Available-for-sale Securities, Amortized Cost Basis	$ 67,171

No authoritative reference available.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27357-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 21

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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Balance Type:	credit
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