Senior and Subordinated Debt (Detail) - Senior and Subordinated Debt - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Senior and Subordinated Debt [Line Items]
Total junior subordinated debentures		$ 47,818	$ 47,606
Total senior and subordinated debt		$ 201,208	200,869
Senior Notes
Senior and Subordinated Debt [Line Items]
Issuance Date		Mar. 21, 2006
Maturity Date		Nov. 22, 2016
Senior notes		$ 114,891	$ 114,768
Interest Rate		5.875%	5.875%
Subordinated Notes
Senior and Subordinated Debt [Line Items]
Issuance Date		Sep. 24, 2009
Maturity Date		Apr. 01, 2016
Subordinated notes		$ 38,499	$ 38,495
Interest Rate		5.85%	5.85%
First Midwest Capital Trust I | 6.950% Junior Subordinated Debentures due December 2033
Senior and Subordinated Debt [Line Items]
Issuance Date		Nov. 11, 2003
Maturity Date		Dec. 01, 2033
Total junior subordinated debentures		$ 37,799	$ 37,797
Interest Rate		6.95%	6.95%
Great Lakes Statutory Trust II
Senior and Subordinated Debt [Line Items]
Acquisition discount		$ 1,900	$ 2,000
Great Lakes Statutory Trust II | LIBOR plus 1.400% Junior Subordinated Debentures due December 2035
Senior and Subordinated Debt [Line Items]
Issuance Date		Dec. 15, 2005
Maturity Date		Dec. 15, 2035
Total junior subordinated debentures	[1]	$ 4,296	$ 4,202
Basis spread on variable rate	[2]	1.40%	1.40%
Great Lakes Statutory Trust III
Senior and Subordinated Debt [Line Items]
Acquisition discount		$ 2,500	$ 2,600
Great Lakes Statutory Trust III | LIBOR plus 1.700% Junior Subordinated Debentures due September 2037
Senior and Subordinated Debt [Line Items]
Issuance Date		Jun. 15, 2007
Maturity Date		Sep. 15, 2037
Total junior subordinated debentures	[1]	$ 5,723	$ 5,607
Basis spread on variable rate	[2]	1.70%	1.70%

[1] The junior subordinated debentures related to GLST II and GLST III were assumed by the Company through the Great Lakes acquisition. As of December 31, 2015, these amounts include acquisition adjustments which resulted in a discount of $1.9 million to GLST II and $2.5 million to GLST III. The acquisition adjustments totaled $2.0 million and $2.6 million to GLST II and GLST III, respectively, as of December 31, 2014.

[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.