Regulatory and Capital Matters (Detail) - Summary of Capital Ratios - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014
First Midwest Bancorp, Inc.
Total capital to risk-weighted assets:
Actual Capital (in Dollars)	[1]	$ 968,331	$ 884,692
Actual Ratio	[1]	11.15%	11.23%
Adequately Capitalized Capital (in Dollars)	[1]	$ 695,029	$ 630,140
Adequately Capitalized Ratio	[1]	8.00%	8.00%
Tier 1 capital to risk-weighted assets:
Actual Capital (in Dollars)	[1]	$ 893,476	$ 802,483
Actual Ratio	[1]	10.28%	10.19%
Adequately Capitalized Capital (in Dollars)	[1]	$ 521,272	$ 315,070
Adequately Capitalized Ratio	[1]	6.00%	4.00%
Tier 1 common capital to risk weighted assets:
Actual Capital (in Dollars)	[1]	$ 845,640
Actual Ratio	[1]	9.73%
Adequately Capitalized Capital (in Dollars)	[1]	$ 390,954
Adequately Capitalized Ratio	[1]	4.50%
Tier 1 leverage to average assets:
Actual Capital (in Dollars)	[1]	$ 893,476	$ 802,483
Actual Ratio	[1]	9.40%	9.03%
Adequately Capitalized Capital (in Dollars)	[1]	$ 380,043	$ 355,362
Adequately Capitalized Ratio	[1]	4.00%	4.00%
First Midwest Bank
Total capital to risk-weighted assets:
Actual Capital (in Dollars)	[1]	$ 929,167	$ 931,829
Actual Ratio	[1]	11.02%	12.30%
Adequately Capitalized Capital (in Dollars)	[1]	$ 674,380	$ 606,038
Adequately Capitalized Ratio	[1]	8.00%	8.00%
To Be Well-Capitalized Under Prompt Corrective Action Provisions Capital (in Dollars)	[1]	$ 842,974	$ 757,547
To Be Well-Capitalized Under Prompt Corrective Action Provisions Ratio	[1]	10.00%	10.00%
Tier 1 capital to risk-weighted assets:
Actual Capital (in Dollars)	[1]	$ 854,322	$ 857,362
Actual Ratio	[1]	10.13%	11.32%
Adequately Capitalized Capital (in Dollars)	[1]	$ 505,785	$ 303,019
Adequately Capitalized Ratio	[1]	6.00%	4.00%
To Be Well-Capitalized Under Prompt Corrective Action Provisions Capital (in Dollars)	[1]	$ 674,380	$ 454,528
To Be Well-Capitalized Under Prompt Corrective Action Provisions	[1]	8.00%	6.00%
Tier 1 common capital to risk weighted assets:
Actual Capital (in Dollars)	[1]	$ 854,322
Actual Ratio	[1]	10.13%
Adequately Capitalized Capital (in Dollars)	[1]	$ 379,338
Adequately Capitalized Ratio	[1]	4.50%
To Be Well-Capitalized Under Prompt Corrective Action Provisions Capital (in Dollars)	[1]	$ 547,933
To Be Well-Capitalized Under Prompt Corrective Action Provisions	[1]	6.50%
Tier 1 leverage to average assets:
Actual Capital (in Dollars)	[1]	$ 854,322	$ 857,362
Actual Ratio	[1]	9.09%	9.76%
Adequately Capitalized Capital (in Dollars)	[1]	$ 375,950	$ 351,222
Adequately Capitalized Ratio	[1]	4.00%	4.00%
To Be Well-Capitalized Under Prompt Corrective Action Provisions Capital (in Dollars)	[1]	$ 469,937	$ 439,028
To Be Well-Capitalized Under Prompt Corrective Action Provisions	[1]	5.00%	5.00%

[1] Basel III Capital Rules, which became effective for the Company on January 1, 2015, revised the risk-based capital requirements and introduced a new capital measure, Tier 1 common capital to risk-weighted assets. As a result, ratios as of December 31, 2015 are computed using the new rules and ratios as of December 31, 2014 are computed using the regulatory guidance applicable at that time.