Loans (Activity for Accretable Yield) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement Schedule [Roll Forward]
Balance at beginning of period	$ 1,039,971	$ 298,257	$ 137,961
Addition attributable to acquisitions		862,127	228,160
Additions (reductions)		(11,354)	(8,450)
Accretion recognized in interest income	(242,169)	(131,559)	(59,414)
Net reclassification (to) from nonaccretable difference	129,095	22,500	0
Balance at end of period	926,897	1,039,971	298,257
Talmer
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement Schedule [Roll Forward]
Balance at beginning of period	798,210	0	0
Addition attributable to acquisitions		862,127	0
Additions (reductions)		0	0
Accretion recognized in interest income	(175,678)	(63,917)	0
Net reclassification (to) from nonaccretable difference	108,821	0	0
Balance at end of period	731,353	798,210	0
Lake Michigan
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement Schedule [Roll Forward]
Balance at beginning of period	121,416	152,999	0
Addition attributable to acquisitions		0	190,246
Additions (reductions)		(3,552)	(12,991)
Accretion recognized in interest income	(29,077)	(33,031)	(24,256)
Net reclassification (to) from nonaccretable difference	2,785	5,000	0
Balance at end of period	95,124	121,416	152,999
Monarch
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement Schedule [Roll Forward]
Balance at beginning of period	27,182	34,558	0
Addition attributable to acquisitions		0	37,914
Additions (reductions)		(1,908)	1,336
Accretion recognized in interest income	(4,533)	(5,468)	(4,692)
Net reclassification (to) from nonaccretable difference	(153)	0	0
Balance at end of period	22,496	27,182	34,558
Northwestern
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement Schedule [Roll Forward]
Balance at beginning of period	69,847	82,623	104,675
Addition attributable to acquisitions		0	0
Additions (reductions)		(6,985)	(3,396)
Accretion recognized in interest income	(20,318)	(15,791)	(18,656)
Net reclassification (to) from nonaccretable difference	11,285	10,000	0
Balance at end of period	60,814	69,847	82,623
OAK
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement Schedule [Roll Forward]
Balance at beginning of period	23,316	28,077	33,286
Addition attributable to acquisitions		0	0
Additions (reductions)		1,091	6,601
Accretion recognized in interest income	(12,563)	(13,352)	(11,810)
Net reclassification (to) from nonaccretable difference	6,357	7,500	0
Balance at end of period	$ 17,110	$ 23,316	$ 28,077