Loans - Activity for Accretable Yield (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Activity for accretable yield
Balance at beginning of period	$ 780,764	$ 1,038,219	$ 926,897	$ 1,039,971
Accretion recognized in interest income	(54,273)	(60,403)	(169,734)	(183,804)
Net reclassification (to) from nonaccretable difference	21,068	(1,812)	(9,604)	119,837
Balance at end of period	747,559	976,004	747,559	976,004
Talmer
Activity for accretable yield
Balance at beginning of period	616,168	801,369	731,353	798,210
Accretion recognized in interest income	(40,695)	(43,816)	(125,471)	(133,478)
Net reclassification (to) from nonaccretable difference	20,250	11,861	(10,159)	104,682
Balance at end of period	595,723	769,414	595,723	769,414
Lake Michigan
Activity for accretable yield
Balance at beginning of period	80,442	121,572	95,124	121,416
Accretion recognized in interest income	(5,980)	(7,201)	(19,040)	(22,050)
Net reclassification (to) from nonaccretable difference	3,108	(14,482)	1,486	523
Balance at end of period	77,570	99,889	77,570	99,889
Monarch
Activity for accretable yield
Balance at beginning of period	20,131	24,270	22,496	27,182
Accretion recognized in interest income	(919)	(1,119)	(3,037)	(3,459)
Net reclassification (to) from nonaccretable difference	(376)	168	(623)	(404)
Balance at end of period	18,836	23,319	18,836	23,319
North-western
Activity for accretable yield
Balance at beginning of period	49,731	71,212	60,814	69,847
Accretion recognized in interest income	(4,341)	(5,263)	(13,863)	(15,222)
Net reclassification (to) from nonaccretable difference	(765)	(1,358)	(2,326)	9,966
Balance at end of period	44,625	64,591	44,625	64,591
OAK
Activity for accretable yield
Balance at beginning of period	14,292	19,796	17,110	23,316
Accretion recognized in interest income	(2,338)	(3,004)	(8,323)	(9,595)
Net reclassification (to) from nonaccretable difference	(1,149)	1,999	2,018	5,070
Balance at end of period	$ 10,805	$ 18,791	$ 10,805	$ 18,791