Senior and Subordinated Debt (Details) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 2016	Dec. 31, 2015	Apr. 01, 2016
Senior and Subordinated Debt [Line Items]
Total junior subordinated debentures		$ 47,924	$ 47,818
Total senior and subordinated debt		$ 162,876	$ 201,208
Senior notes
Senior and Subordinated Debt [Line Items]
Issuance Date		Nov. 22, 2011
Maturity Date		Nov. 22, 2016
Interest Rate		5.875%	5.875%
Senior notes		$ 114,952	$ 114,891
Maturing long-term debt		$ 115,000
Subordinated notes
Senior and Subordinated Debt [Line Items]
Issuance Date		Mar. 01, 2006
Maturity Date		Apr. 01, 2016
Interest Rate			5.85%	5.85%
Subordinated debt		$ 0	$ 38,499	$ 38,500
Junior Subordinated Debentures, First Midwest Capital Trust I | First Midwest Capital Trust I (FMCT)
Senior and Subordinated Debt [Line Items]
Issuance Date		Nov. 11, 2003
Maturity Date		Dec. 01, 2033
Interest Rate		6.95%	6.95%
Total junior subordinated debentures		$ 37,799	$ 37,799
Junior Subordinated Debentures Great Lakes Statutory Trust II | Great Lakes Statutory Trust II (GLST II)
Senior and Subordinated Debt [Line Items]
Issuance Date		Dec. 15, 2005
Maturity Date		Dec. 15, 2035
Basis spread on variable rate	[1],[2]	1.40%	1.40%
Total junior subordinated debentures	[2]	$ 4,344	$ 4,296
Acquisition discount		$ 1,800	$ 1,900
Junior Subordinated Debentures, Great Lakes Statutory Trust III | Great Lakes Statutory Trust III (GLST III)
Senior and Subordinated Debt [Line Items]
Issuance Date		Jun. 21, 2007
Maturity Date		Sep. 21, 2037
Basis spread on variable rate	[1],[2]	1.70%	1.70%
Total junior subordinated debentures	[2]	$ 5,781	$ 5,723
Acquisition discount		$ 2,500	$ 2,500

[1] The interest rates are a variable rate based on the three-month LIBOR plus 1.400% and 1.700% for GLST II and GLST III, respectively.

[2] The junior subordinated debentures related to GLST II and GLST III were assumed by the Company during 2014 through the acquisition of Great Lakes Financial Resources, Inc., the holding company for Great Lakes Bank. These amounts include acquisition adjustments which resulted in a discount of $1.8 million to GLST II and $2.5 million to GLST III as of June 30, 2016 and $1.9 million to GLST II and $2.5 million to GLST III as of December 31, 2015.