Senior and Subordinated Debt (Details) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2016	Dec. 31, 2015	Sep. 29, 2016	Apr. 01, 2016
Debt Instrument [Line Items]
Senior notes		$ 114,983	$ 114,891
Subordinated notes		146,484	38,499
Total junior subordinated debentures		47,977	47,818
Senior and subordinated debt		$ 309,444	$ 201,208
Senior notes
Debt Instrument [Line Items]
Issuance Date		Nov. 22, 2011
Maturity Date		Nov. 22, 2016
Interest Rate		5.875%	5.875%
Maturing long-term debt		$ 115,000
Subordinated notes due in 2026
Debt Instrument [Line Items]
Issuance Date		Sep. 29, 2016
Maturity Date		Sep. 29, 2026
Interest Rate		5.875%		5.875%
Proceeds from Issuance of Debt		$ 146,484
Debt Instrument, Face Amount				$ 150,000
Subordinated notes		$ 146,484	$ 0
Subordinated notes due in 2016
Debt Instrument [Line Items]
Issuance Date		Mar. 01, 2006
Maturity Date		Apr. 01, 2016
Interest Rate			5.85%		5.85%
Subordinated notes		$ 0	$ 38,499		$ 38,500
Junior Subordinated Debentures, First Midwest Capital Trust I | First Midwest Capital Trust I (FMCT)
Debt Instrument [Line Items]
Issuance Date		Nov. 11, 2003
Maturity Date		Dec. 01, 2033
Interest Rate		6.95%	6.95%
Total junior subordinated debentures		$ 37,800	$ 37,799
Junior Subordinated Debentures, Great Lakes Statutory Trust II | Great Lakes Statutory Trust II (GLST II)
Debt Disclosure [Abstract]
Debt Instrument, Unamortized Discount		$ 1,800	$ 1,900
Debt Instrument [Line Items]
Issuance Date	[1]	Dec. 15, 2005
Maturity Date	[1]	Dec. 15, 2035
Basis Spread on Variable Rate	[1],[2]	1.40%	1.40%
Total junior subordinated debentures	[1]	$ 4,367	$ 4,296
Junior Subordinated Debentures, Great Lakes Statutory Trust III | Great Lakes Statutory Trust III (GLST III)
Debt Disclosure [Abstract]
Debt Instrument, Unamortized Discount		$ 2,400	$ 2,500
Debt Instrument [Line Items]
Issuance Date	[1]	Jun. 21, 2007
Maturity Date	[1]	Sep. 21, 2037
Basis Spread on Variable Rate	[1],[2]	1.70%	1.70%
Total junior subordinated debentures	[1]	$ 5,810	$ 5,723

[1] 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 $2.4 million to GLST III as of September 30, 2016 and $1.9 million to GLST II and $2.5 million to GLST III as of December 31, 2015.

[2] 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.