Acquisitions (Textual) (Details) (USD $)	0 Months Ended
	Jan. 05, 2015	Apr. 01, 2015	Oct. 31, 2014	Dec. 07, 2012 Branches	Apr. 30, 2010 Offices	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Business Acquisition [Line Items]
Goodwill						$ 180,128,000	$ 180,128,000	$ 120,164,000
Lake Michigan Financial Corp [Member]
Business Acquisition [Line Items]
Business Acquisition, Share Price	$ 16.64
Exchange Ratio of Common Stock Issued, Business Combinations	1.326
Acquisition purchase price	184,100,000
Monarch Community Bancorp [Member]
Business Acquisition [Line Items]
Exchange Ratio of Common Stock Issued, Business Combinations		0.0982
Business Combination, Acquired Receivables, Fair Value						128,000,000
Acquisition purchase price		27,200,000
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares		861,000
Assets of acquiree company						170,000,000
Deposits of acquiree company						143,000,000
Northwestern Bancorp [Member]
Business Acquisition [Line Items]
Expected Principal Cash Flows of Contractually Required Acquired Loan Portfolio			481,000,000
Expected Interest Cash Flows of Contractually Required Acquired Loan Portfolio			104,000,000
Business Combination, Acquired Receivables, Fair Value			475,000,000
Certain Loans Acquired in Transfer, Nonaccretable Difference			34,000,000
Acquisition purchase price			121,000,000
Goodwill			60,000,000
Intangibles acquired			12,900,000
Service provided by acquiree bank through number of branches			25
Assets of acquiree company			815,000,000
Deposits of acquiree company			794,000,000
Outstanding contractual principal balance of acquired loan portfolio						453,000,000	485,000,000
Carrying amount of acquired loan portfolio						421,000,000	452,000,000
Business Combination, Acquired Receivables, Gross Contractual Amount			507,000,000
Interest Payments Receivable of Acquired Loan Portfolio			112,000,000
Business Combination, Acquired Receivables, Estimated Uncollectible			26,000,000
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield, Disposals of Loans			110,000,000
Independent Bank [Member]
Business Acquisition [Line Items]
Number of branches acquired (branches)				21
Deposits acquired				404,000,000
Loans acquired				44,000,000
Acquisition purchase price				8,100,000
Premium paid on deposits purchased				11,500,000
Premium paid as a percentage of deposits purchased (percent)				2.85%
Discount on loans purchased (percent)				1.75%
Goodwill				6,800,000
Independent Bank [Member] | Core Deposits [Member]
Business Acquisition [Line Items]
Intangibles acquired				5,600,000
OAK [Member]
Business Acquisition [Line Items]
Loans acquired					627,000,000
Acquisition purchase price					83,700,000
Service provided by acquiree bank through number of branches					14
Assets of acquiree company					820,000,000
Deposits of acquiree company					693,000,000
Brokered deposits of acquiree company					193,000,000
Contractually required principal payments of loan portfolio, acquisition date					683,000,000
Acquired loan portfolio acquisition date fair value					627,000,000
Outstanding contractual principal balance of acquired loan portfolio						255,000,000	268,000,000	313,000,000
Carrying amount of acquired loan portfolio						$ 234,000,000	$ 246,000,000	$ 289,000,000