Acquisitions (Textual) (Details) $ / shares in Units, $ in Thousands							3 Months Ended	9 Months Ended
		May. 31, 2015 USD ($) Offices $ / shares shares	Apr. 01, 2015 USD ($) shares	Oct. 31, 2014 USD ($) Offices	Dec. 07, 2012 USD ($) Branches	Apr. 30, 2010 USD ($) Offices	Sep. 30, 2015 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Goodwill							$ 286,454	$ 286,454	$ 120,164	$ 180,128
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield, Disposals of Loans								228,160
Lake Michigan Financial Corp [Member]
Business Acquisition [Line Items]
Business Combination, Provisional Information, Initial Accounting Incomplete, Deferred Taxes from Tax-Credit Projects							200
Business Combination, Provision Information, Initial Accounting Incomplete, Goodwill							876
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Property, Plant, and Equipment							(1,689)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Intangibles							700
Expected Principal Cash Flows of Contractually Required Acquired Loan Portfolio		$ 986,100
Expected Interest Cash Flows of Contractually Required Acquired Loan Portfolio		$ 189,600
Business Acquisition, Share Price | $ / shares		$ 16.64
Exchange Ratio of Common Stock Issued, Business Combinations		1.326
Business Combination, Acquired Receivables, Fair Value		$ 985,500
Certain Loans Acquired in Transfer, Nonaccretable Difference		22,600
Acquisition purchase price		$ 187,394
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares | shares		4,322,101
Goodwill		$ 100,878
Intangibles acquired		$ 8,000
Service provided by acquiree bank through number of branches | Offices		5
Assets of acquiree company		$ 1,240,000
Purchase Accounting Fair Value Adjustment, Core Deposit Intangible Asset	[1]	(8,003)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill		8,600
Business Combination, Pro Forma Information, Net Interest Income of Acquiree since Acquisition Date							9,700	12,900
Business Combination, Pro Forma Information, Noninterest Income of Acquiree since Acquisition Date							500	800
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual							3,600	4,800
Business Combination, Separately Recognized Transactions, Additional Disclosures, Acquisition Cost Expensed							900	3,400
Outstanding contractual principal balance of acquired loan portfolio							914,000	914,000
Carrying amount of acquired loan portfolio							891,000	891,000
Business Combination, Acquired Receivables, Gross Contractual Amount		1,010,000
Interest Payments Receivable of Acquired Loan Portfolio		190,200
Business Combination, Acquired Receivables, Estimated Uncollectible		22,000
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield, Disposals of Loans		190,200						190,246
Business Combination, Consideration Transferred, Value of Stock and Options Issued		132,900
Business Combination, Consideration Transferred, Cash Paid for Outstanding Stock		$ 54,478
Business Combination, Provisional Information, Initial Accounting Incomplete, Other Assets							(790)
Business Combination, Provisional Information, Initial Accounting Incomplete, Deferred Income Tax Assets							903
Monarch Community Bancorp [Member]
Business Acquisition [Line Items]
Business Combination, Provision Information, Initial Accounting Incomplete, Goodwill							(100)
Expected Principal Cash Flows of Contractually Required Acquired Loan Portfolio			$ 122,600
Expected Interest Cash Flows of Contractually Required Acquired Loan Portfolio			$ 37,100
Exchange Ratio of Common Stock Issued, Business Combinations			0.0982
Business Combination, Acquired Receivables, Fair Value			$ 121,783
Certain Loans Acquired in Transfer, Nonaccretable Difference			7,100
Acquisition purchase price			$ 27,191
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares | shares			860,575
Goodwill			$ 5,314
Intangibles acquired			1,930
Assets of acquiree company			183,000
Deposits of acquiree company			144,311
Purchase Accounting Fair Value Adjustment, Core Deposit Intangible Asset			(1,930)
Business Combination, Separately Recognized Transactions, Additional Disclosures, Acquisition Cost Expensed								2,300
Outstanding contractual principal balance of acquired loan portfolio							119,000	119,000
Carrying amount of acquired loan portfolio							112,000	112,000
Business Combination, Acquired Receivables, Gross Contractual Amount			128,900
Interest Payments Receivable of Acquired Loan Portfolio			37,800
Business Combination, Acquired Receivables, Estimated Uncollectible			6,300
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield, Disposals of Loans			37,900					37,914
Business Combination, Consideration Transferred, Cash Paid for Outstanding Stock			203
Operating Loss Carryforwards			22,600
Tax Credit Carryforwards Not Expected to Be Utilized			$ 1,700
Business Combination, Provisional Information, Initial Accounting Incomplete, Deferred Income Tax Assets							100
Northwestern Bancorp [Member]
Business Acquisition [Line Items]
Business Combination, Provision Information, Initial Accounting Incomplete, Goodwill							100
Expected Principal Cash Flows of Contractually Required Acquired Loan Portfolio				$ 481,000
Expected Interest Cash Flows of Contractually Required Acquired Loan Portfolio				104,000
Business Combination, Acquired Receivables, Fair Value				475,000
Certain Loans Acquired in Transfer, Nonaccretable Difference				34,000
Acquisition purchase price				121,000
Goodwill				60,100
Intangibles acquired				$ 12,900
Business Combination, Provisional Information, Initial Accounting Incomplete, Other Real Estate Owned							300
Business Combination, Provisional Information, Initial Accounting Incomplete, Federal Income Taxes Receivable							100
Service provided by acquiree bank through number of branches | Offices				25
Assets of acquiree company				$ 815,000
Deposits of acquiree company				794,000
Outstanding contractual principal balance of acquired loan portfolio							379,000	379,000		485,000
Carrying amount of acquired loan portfolio							347,000	347,000		452,000
Business Combination, Acquired Receivables, Gross Contractual Amount				507,000
Interest Payments Receivable of Acquired Loan Portfolio				112,000
Business Combination, Acquired Receivables, Estimated Uncollectible				26,000
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield, Disposals of Loans				$ 110,000				0
Business Combination, Provisional Information, Initial Accounting Incomplete, Deferred Income Tax Assets							200
Independent Bank [Member]
Business Acquisition [Line Items]
Number of branches acquired (branches) | Branches					21
Deposits acquired					$ 404,000
Loans acquired					44,000
Acquisition purchase price					8,100
Premium paid on deposits purchased					$ 11,500
Premium paid as a percentage of deposits purchased (percent)					2.85%
Discount on loans purchased (percent)					1.75%
Goodwill					$ 6,800
Independent Bank [Member] | Core Deposits [Member]
Business Acquisition [Line Items]
Intangibles acquired					$ 5,600
OAK [Member]
Business Acquisition [Line Items]
Loans acquired						$ 627,000
Acquisition purchase price						$ 83,700
Service provided by acquiree bank through number of branches | Offices						14
Assets of acquiree company						$ 820,000
Deposits of acquiree company						693,000
Brokered deposits of acquiree company						193,000
Contractually required principal payments of loan portfolio, acquisition date						683,000
Acquired loan portfolio acquisition date fair value						$ 627,000
Outstanding contractual principal balance of acquired loan portfolio							219,000	219,000	286,000	268,000
Carrying amount of acquired loan portfolio							$ 198,000	198,000	263,000	$ 246,000
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield, Disposals of Loans								$ 0	$ 0

[1] Includes adjustments to the fair value as a result of additional valuation information obtained during the third quarter of 2015, including the corresponding tax effects. A summary of these adjustments is included in the section "Allocation of Purchase Price" within this footnote.