Acquisitions Activity for Acquired Loans (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Activity for accretable yield includes contractually due interest of acquired loans
Balance at beginning of year	$ 32,610		$ 49,390
Accretable yield added during the year from acquisitions	110,003		0
Accretable Yield Additions, Net of Reductions	3,806	[1]	(29)	[1]
Accretion recognized into interest income	(18,458)		(16,876)
Reclassification from nonaccretable difference	10,000		125
Balance at end of year	137,961		32,610
Northwestern Bancorp [Member]
Activity for accretable yield includes contractually due interest of acquired loans
Balance at beginning of year	0
Accretable yield added during the year from acquisitions	110,003
Accretable Yield Additions, Net of Reductions	(1,605)	[1]
Accretion recognized into interest income	(3,723)
Reclassification from nonaccretable difference	0
Balance at end of year	104,675
OAK [Member]
Activity for accretable yield includes contractually due interest of acquired loans
Balance at beginning of year	32,610		49,390
Accretable yield added during the year from acquisitions	0		0
Accretable Yield Additions, Net of Reductions	5,411	[1]	(29)	[1]
Accretion recognized into interest income	(14,735)		(16,876)
Reclassification from nonaccretable difference	10,000		125
Balance at end of year	$ 33,286		$ 32,610

[1] Represents additions in estimated contractual interest expected to be collected from acquired loans being renewed or extended, less reductions in contractual interest resulting from the early payoff of acquired loans.