Loans - Activity for Accretable Yield (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Activity for accretable yield
Balance at beginning of period	$ 926,897	$ 1,039,971
Accretion recognized in interest income	(58,561)	(60,187)
Net reclassification (to) from nonaccretable difference	362	20,624
Balance at end of period	868,698	1,000,408
Talmer
Activity for accretable yield
Balance at beginning of period	731,353	798,210
Accretion recognized in interest income	(42,640)	(44,571)
Net reclassification (to) from nonaccretable difference	(2,883)	21,139
Balance at end of period	685,830	774,778
Lake Michigan
Activity for accretable yield
Balance at beginning of period	95,124	121,416
Accretion recognized in interest income	(6,758)	(7,266)
Net reclassification (to) from nonaccretable difference	1,790	(939)
Balance at end of period	90,156	113,211
Monarch
Activity for accretable yield
Balance at beginning of period	22,496	27,182
Accretion recognized in interest income	(1,156)	(1,181)
Net reclassification (to) from nonaccretable difference	(186)	54
Balance at end of period	21,154	26,055
North-western
Activity for accretable yield
Balance at beginning of period	60,814	69,847
Accretion recognized in interest income	(4,904)	(3,892)
Net reclassification (to) from nonaccretable difference	(510)	(1,058)
Balance at end of period	55,400	64,897
OAK
Activity for accretable yield
Balance at beginning of period	17,110	23,316
Accretion recognized in interest income	(3,103)	(3,277)
Net reclassification (to) from nonaccretable difference	2,151	1,428
Balance at end of period	$ 16,158	$ 21,467