Mergers and Acquisitions (Assets Acquired and Liabilities Assumed) (Details) - USD ($) $ in Thousands				12 Months Ended
	Aug. 31, 2016	May 31, 2015	Apr. 01, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017
Investment securities:
Goodwill				$ 1,133,534		$ 1,134,568
Bank-owned life insurance				143,700		147,600
Fair value of liabilities assumed:
Short-term borrowings				825,000		2,000,000
Long-term borrowings				597,847		$ 372,882
Talmer
Consideration paid:
Stock	$ 1,504,811
Cash	107,638
Total consideration	1,612,449
Fair value of identifiable assets acquired:
Cash and cash equivalents	433,352
Investment securities:
Available-for-sale	808,894
Held-to-maturity	1,657
Loans held-for-sale	244,916
Premises and equipment	38,793
Loan servicing rights	42,462
Other intangible assets	19,088
Interest receivable and other assets	395,539
Goodwill	847,701
Loans	4,882,402
Total identifiable assets acquired	6,867,103
Fair value of liabilities assumed:
Noninterest-bearing deposits	1,236,902
Interest-bearing deposits	4,057,716
Interest payable and other liabilities	100,936
Securities sold under agreements to repurchase with customers	19,704
Short-term borrowings	387,500
Long-term borrowings	299,597
Total liabilities assumed	6,102,355
Fair value of net identifiable assets acquired	$ 764,748
Lake Michigan
Consideration paid:
Stock		$ 132,916
Cash		54,478
Total consideration		187,394		187,394
Fair value of identifiable assets acquired:
Cash and cash equivalents				39,301
Investment securities				66,699
Investment securities:
Loans		985,500		985,542
Premises and equipment				10,975
Other intangible assets				8,003
Deferred tax asset, net				16,715
Goodwill		101,061		101,061
Bank-owned life insurance				23,844
Loans				37,695
Total identifiable assets acquired				1,289,835
Fair value of liabilities assumed:
Deposits		924,700		924,697
Short-term borrowings				30,000
Long-term borrowings				124,857
Other liabilities				22,887
Total liabilities assumed				$ 1,102,441
Fair value of net identifiable assets acquired		1,240,000
Shareholders' equity		89,280
Loans		(22,600)
Allowance for loan losses		15,888
Deferred tax assets, net		4,096
Premises and equipment		(5,031)
Core deposit intangibles		8,003
Deposits and borrowings, net		(3,182)
Other assets and other liabilities		(121)
Fair value of adjusted net assets acquired		$ 86,333
Monarch
Consideration paid:
Stock			$ 26,988
Cash			203
Total consideration			27,191		$ 27,191
Fair value of identifiable assets acquired:
Cash and cash equivalents					32,171
Investment securities:
Loans					121,783
Premises and equipment					3,019
Other intangible assets					1,930
Net operating loss carryforward					7,900
Deferred tax asset, net					2,392
Goodwill			5,339		5,339
Mortgage servicing rights					1,284
Loans					6,972
Total identifiable assets acquired					182,790
Fair value of liabilities assumed:
Deposits			144,300		144,300
Long-term borrowings					8,000
Other liabilities					3,299
Total liabilities assumed					$ 155,599
Fair value of net identifiable assets acquired			182,800
Shareholders' equity			15,270
Loans			(7,150)
Allowance for loan losses			2,128
Net operating loss carryforward			7,900
Other			1,826
Premises and equipment			(415)
Core deposit intangibles			1,930
Mortgage servicing rights			315
Other assets and other liabilities			48
Fair value of adjusted net assets acquired			$ 21,852