Loans - Activity for Accretable Yield (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Activity for accretable yield
Balance at beginning of period	$ 602,664	$ 868,698	$ 647,325	$ 926,897
Accretion recognized in interest income	(47,421)	(56,900)	(96,566)	(115,461)
Net reclassification (to) from nonaccretable difference	3,547	(31,034)	8,031	(30,672)
Balance at end of period	558,790	780,764	558,790	780,764
Talmer
Activity for accretable yield
Balance at beginning of period	469,713	685,830	505,332	731,353
Accretion recognized in interest income	(36,730)	(42,136)	(74,761)	(84,776)
Net reclassification (to) from nonaccretable difference	730	(27,526)	3,142	(30,409)
Balance at end of period	433,713	616,168	433,713	616,168
Lake Michigan
Activity for accretable yield
Balance at beginning of period	68,995	90,156	73,132	95,124
Accretion recognized in interest income	(5,291)	(6,302)	(10,842)	(13,060)
Net reclassification (to) from nonaccretable difference	409	(3,412)	1,823	(1,622)
Balance at end of period	64,113	80,442	64,113	80,442
Monarch
Activity for accretable yield
Balance at beginning of period	16,967	21,154	17,832	22,496
Accretion recognized in interest income	(852)	(962)	(1,626)	(2,118)
Net reclassification (to) from nonaccretable difference	750	(61)	659	(247)
Balance at end of period	16,865	20,131	16,865	20,131
North-western
Activity for accretable yield
Balance at beginning of period	38,644	55,400	41,455	60,814
Accretion recognized in interest income	(3,214)	(4,618)	(6,634)	(9,522)
Net reclassification (to) from nonaccretable difference	745	(1,051)	1,354	(1,561)
Balance at end of period	36,175	49,731	36,175	49,731
OAK
Activity for accretable yield
Balance at beginning of period	8,345	16,158	9,574	17,110
Accretion recognized in interest income	(1,334)	(2,882)	(2,703)	(5,985)
Net reclassification (to) from nonaccretable difference	913	1,016	1,053	3,167
Balance at end of period	$ 7,924	$ 14,292	$ 7,924	$ 14,292