Senior and Subordinated Debt - Schedule of Senior and Subordinated Debt (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total junior subordinated debentures		$ 56,526	$ 48,243
Senior and subordinated debt		203,808	195,170
Debt instrument, unamortized discount		6,000
GLST II and GLST III
Debt Instrument [Line Items]
Debt instrument, unamortized discount			4,000
Subordinated notes due in 2026
Debt Instrument [Line Items]
Subordinated notes		$ 147,282	146,927
Issuance date		Sep. 29, 2016
Maturity date		Sep. 29, 2026
Interest rate		5.875%
Junior Subordinated Debentures, First Midwest Capital Trust I | First Midwest Capital Trust I (FMCT)
Debt Instrument [Line Items]
Total junior subordinated debentures		$ 37,803	37,801
Issuance date		Nov. 11, 2003
Maturity date		Dec. 01, 2033
Interest rate		6.95%
Junior Subordinated Debentures, Great Lakes Statutory Trust II | Great Lakes Statutory Trust II (GLST II)
Debt Instrument [Line Items]
Total junior subordinated debentures	[1]	$ 4,580	$ 4,486
Issuance date	[1]	Dec. 15, 2005
Maturity date	[1]	Dec. 15, 2035
Basis spread on variable interest rate	[1],[2]	1.40%	1.40%
Junior Subordinated Debentures, Great Lakes Statutory Trust III | Great Lakes Statutory Trust III (GLST III)
Debt Instrument [Line Items]
Total junior subordinated debentures	[1]	$ 6,071	$ 5,956
Issuance date	[1]	Jun. 15, 2007
Maturity date	[1]	Sep. 15, 2037
Basis spread on variable interest rate	[1],[2]	1.70%	1.70%
Junior Subordinated Debentures, Northern States Statutory Trust I | Northern States Statutory Trust I (NSST I)
Debt Instrument [Line Items]
Total junior subordinated debentures	[1]	$ 8,072	$ 0
Issuance date	[1]	Sep. 07, 2005
Maturity date	[1]	Oct. 07, 2035
Basis spread on variable interest rate	[1],[2]	1.80%

[1] The junior subordinated debentures related to GLST II, GLST III, and NSST I were assumed by the Company through acquisitions. These amounts include acquisition adjustment discounts that total $6.0 million and $4.0 million as of December 31, 2018 and 2017, respectively.

[2] The interest rates are a variable rate, based on three-month LIBOR plus 1.400%, 1.700% and $1.800% for GLST II, GLST III, and NSST I, respectively.