Subsequent Event (Details Textual) - USD ($)		Apr. 20, 2016	Mar. 31, 2016	Dec. 31, 2015
Subsequent Event
Total assets			$ 9,120,742,000	$ 9,089,232,000
Gross loans	[1]		$ 5,197,193,000	$ 5,078,681,000
Treasure State Bank
Subsequent Event
Subsequent Event, Description		the Company announced the signing of a definitive agreement to acquire Treasure State Bank, a community bank based in Missoula, Montana
Total assets		$ 71,000,000
Gross loans		53,000,000
Total deposits		$ 58,000,000

[1] Includes net deferred fees, costs, premiums and discounts of $14,010,000 and $15,529,000 at March 31, 2016 and December 31, 2015, respectively.