Regulatory Capital - Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Total risk-based capital:
Actual Amount	$ 1,254.8	$ 1,112.5
Actual Ratio	12.75978%	12.75962%
Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 971.1	$ 806.5
Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	9.875%	9.25%
Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 1,032.6	$ 915.5
Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	10.50%	10.50%
Well Capitalized Amount	$ 983.4	$ 871.9
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	$ 1,181.2	$ 1,040.3
Actual Ratio	12.01132%	11.93154%
Tier One Risk Based Capital under Basel III Phase-in Schedule	$ 774.5	$ 632.1
Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	7.875%	7.25%
Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 835.9	$ 741.1
Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	8.50%	8.50%
Well Capitalized Amount	$ 786.7	$ 697.5
Well Capitalized Ratio	8.00%	8.00%
Common equity tier one risk-based capital:
Actual Amount	$ 1,096.8	$ 962.4
Actual Ratio	11.15322%	11.03808%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 626.9	$ 501.3
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	6.375%	5.75%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 688.4	$ 610.3
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	7.00%	7.00%
Well Capitalized Amount	$ 639.2	$ 566.7
Well Capitalized Ratio	6.50%	6.50%
Leverage capital ratio:
Actual Amount	$ 1,181.2	$ 1,040.3
Actual Ratio	9.72992%	8.85532%
Adequately Capitalized Amount	$ 485.6	$ 469.9
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	$ 607.0	$ 587.4
Well Capitalized Ratio	5.00%	5.00%
FIB
Total risk-based capital:
Actual Amount	$ 1,107.7	$ 1,066.6
Actual Ratio	12.28251%	12.29128%
Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 890.5	$ 802.7
Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	9.875%	9.25%
Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 946.9	$ 911.2
Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	10.50%	10.50%
Well Capitalized Amount	$ 901.8	$ 867.8
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	$ 1,034.0	$ 994.4
Actual Ratio	11.46631%	11.45926%
Tier One Risk Based Capital under Basel III Phase-in Schedule	$ 710.2	$ 629.1
Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	7.875%	7.25%
Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 766.5	$ 737.6
Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	8.50%	8.50%
Well Capitalized Amount	$ 721.5	$ 694.2
Well Capitalized Ratio	8.00%	8.00%
Common equity tier one risk-based capital:
Actual Amount	$ 1,034.0	$ 994.4
Actual Ratio	11.46631%	11.45926%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 574.9	$ 499.0
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	6.375%	5.75%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 631.3	$ 607.4
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	7.00%	7.00%
Well Capitalized Amount	$ 586.2	$ 564.1
Well Capitalized Ratio	6.50%	6.50%
Leverage capital ratio:
Actual Amount	$ 1,034.0	$ 994.4
Actual Ratio	8.76766%	8.48004%
Adequately Capitalized Amount	$ 471.8	$ 469.1
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	$ 589.7	$ 586.3
Well Capitalized Ratio	5.00%	5.00%
Inland Northwest Bank
Total risk-based capital:
Actual Amount	$ 77.6
Actual Ratio	9.86329%
Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 77.7
Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	9.875%
Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 82.7
Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	10.50%
Well Capitalized Amount	$ 78.7
Well Capitalized Ratio	10.00%
Tier 1 risk-based capital:
Actual Amount	$ 77.6
Actual Ratio	9.86329%
Tier One Risk Based Capital under Basel III Phase-in Schedule	$ 62.0
Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	7.875%
Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 66.9
Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	8.50%
Well Capitalized Amount	$ 63.0
Well Capitalized Ratio	8.00%
Common equity tier one risk-based capital:
Actual Amount	$ 77.6
Actual Ratio	9.86329%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 50.2
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	6.375%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 55.1
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	7.00%
Well Capitalized Amount	$ 51.2
Well Capitalized Ratio	6.50%
Leverage capital ratio:
Actual Amount	$ 77.6
Actual Ratio	9.80699%
Adequately Capitalized Amount	$ 31.7
Adequately Capitalized Ratio	4.00%
Well Capitalized Amount	$ 39.6
Well Capitalized Ratio	5.00%