Regulatory and Capital Matters (Details) - Summary of Capital Ratios - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
First Midwest Bancorp, Inc.
Total capital to risk-weighted assets:
Actual Capital	$ 1,225,529	$ 968,331
Actual Ratio (percent)	12.23%	11.15%
Adequately Capitalized Capital	$ 864,176	$ 695,029
Adequately Capitalized Ratio (percent)	8.625%	8.00%
Tier 1 capital to risk-weighted assets:
Actual Capital	$ 991,873	$ 893,476
Actual Ratio (percent)	9.90%	10.28%
Adequately Capitalized Capital	$ 663,788	$ 521,272
Adequately Capitalized Ratio (percent)	6.625%	6.00%
Common equity Tier 1 to risk-weighted assets:
Actual Capital	$ 941,315	$ 845,640
Actual Ratio (percent)	9.39%	9.73%
Adequately Capitalized Capital	$ 513,496	$ 390,954
Adequately Capitalized Ratio (percent)	5.125%	4.50%
Tier 1 capital to average assets:
Actual Capital	$ 991,873	$ 893,476
Actual Ratio (percent)	8.99%	9.40%
Adequately Capitalized Capital	$ 441,473	$ 380,043
Adequately Capitalized Ratio (percent)	4.00%	4.00%
First Midwest Bank
Total capital to risk-weighted assets:
Actual Capital	$ 1,043,482	$ 929,167
Actual Ratio (percent)	10.73%	11.02%
Adequately Capitalized Capital	$ 838,741	$ 674,380
Adequately Capitalized Ratio (percent)	8.625%	8.00%
To Be Well-Capitalized Under Prompt Corrective Action Provisions Capital	$ 972,454	$ 842,974
To Be Well-Capitalized Under Prompt Corrective Action Provisions Ratio (percent)	10.00%	10.00%
Tier 1 capital to risk-weighted assets:
Actual Capital	$ 956,399	$ 854,322
Actual Ratio (percent)	9.83%	10.13%
Adequately Capitalized Capital	$ 644,251	$ 505,785
Adequately Capitalized Ratio (percent)	6.625%	6.00%
To Be Well-Capitalized Under Prompt Corrective Action Provisions Capital	$ 777,963	$ 674,380
To Be Well-Capitalized Under Prompt Corrective Action Provisions Ratio (percent)	8.00%	8.00%
Common equity Tier 1 to risk-weighted assets:
Actual Capital	$ 956,399	$ 854,322
Actual Ratio (percent)	9.83%	10.13%
Adequately Capitalized Capital	$ 498,383	$ 379,338
Adequately Capitalized Ratio (percent)	5.125%	4.50%
To Be Well-Capitalized Under Prompt Corrective Action Provisions Capital	$ 632,095	$ 547,933
To Be Well-Capitalized Under Prompt Corrective Action Provisions Ratio ( percent)	6.50%	6.50%
Tier 1 capital to average assets:
Actual Capital	$ 956,399	$ 854,322
Actual Ratio (percent)	8.76%	9.09%
Adequately Capitalized Capital	$ 436,560	$ 375,950
Adequately Capitalized Ratio (percent)	4.00%	4.00%
To Be Well-Capitalized Under Prompt Corrective Action Provisions Capital	$ 545,700	$ 469,937
To Be Well-Capitalized Under Prompt Corrective Action Provisions Ratio (percent)	5.00%	5.00%