Other Intangible Assets (Activity for Capitalized MSRs) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Other Intangible Assets [Roll Forward]
Total other intangible assets, Balance at beginning of year	$ 13,424
Amortization	(3,545)		(3,492)		(3,969)
Change in valuation allowance attributable to mortage servicing assets			0		0
Total other intangible assets, Balance at end of year	33,080		13,424
Valuation Allowance for Impairment of Recognized Servicing Assets, Balance	200	[1]	0	[1]	0	[1]
Mortgage servicing rights [Member]
Other Intangible Assets [Roll Forward]
Total other intangible assets, Balance at beginning of year	3,423		3,478		3,593
Additions	1,075		1,528		2,285
Amortization	(1,316)		(1,583)		(2,400)
Change in valuation allowance attributable to mortage servicing assets	(200)
Total other intangible assets, Balance at end of year	12,217		3,423		3,478
Mortgage servicing rights [Member] | Northwestern Bancorp [Member]
Other Intangible Assets [Roll Forward]
Additions	9,235
Mortgage servicing rights [Member] | OAK [Member]
Other Intangible Assets [Roll Forward]
Additions			$ 0		$ 0

[1] MSRs are stratified into servicing assets originated by the Corporation and those acquired in acquisitions of other institutions and further stratified into relatively homogeneous pools based on products with similar characteristics. The MSR impairment valuation allowance recorded at December 31, 2014 related to impairment within certain pools attributable to the Corporation's servicing portfolio that was acquired in the Northwestern transaction.