Regulatory Capital - Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Total risk-based capital:
Actual Amount	$ 1,160.9	$ 1,112.5
Actual Ratio	12.97488%	12.75962%
Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 883.6	$ 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	$ 939.5	$ 915.5
Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	10.50%	10.50%
Well Capitalized Amount	$ 894.7	$ 871.9
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	$ 1,086.8	$ 1,040.3
Actual Ratio	12.14683%	11.93154%
Tier One Risk Based Capital under Basel III Phase-in Schedule	$ 704.6	$ 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	$ 760.5	$ 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	$ 715.8	$ 697.5
Well Capitalized Ratio	8.00%	8.00%
Common equity tier one risk-based capital:
Actual Amount	$ 1,006.8	$ 962.4
Actual Ratio	11.25272%	11.03808%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 570.4	$ 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	$ 626.3	$ 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	$ 581.6	$ 566.7
Well Capitalized Ratio	6.50%	6.50%
Leverage capital ratio:
Actual Amount	$ 1,086.8	$ 1,040.3
Actual Ratio	9.28186%	8.85532%
Adequately Capitalized Amount	$ 468.4	$ 469.9
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	$ 585.5	$ 587.4
Well Capitalized Ratio	5.00%	5.00%
FIB
Total risk-based capital:
Actual Amount	$ 1,087.7	$ 1,066.6
Actual Ratio	12.20156%	12.29128%
Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 880.3	$ 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	$ 936.0	$ 911.2
Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	10.50%	10.50%
Well Capitalized Amount	$ 891.4	$ 867.8
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	$ 1,013.6	$ 994.4
Actual Ratio	11.37045%	11.45926%
Tier One Risk Based Capital under Basel III Phase-in Schedule	$ 702.0	$ 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	$ 757.7	$ 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	$ 713.2	$ 694.2
Well Capitalized Ratio	8.00%	8.00%
Common equity tier one risk-based capital:
Actual Amount	$ 1,013.6	$ 994.4
Actual Ratio	11.37045%	11.45926%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 568.3	$ 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	$ 624.0	$ 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	$ 579.4	$ 564.1
Well Capitalized Ratio	6.50%	6.50%
Leverage capital ratio:
Actual Amount	$ 1,013.6	$ 994.4
Actual Ratio	8.68594%	8.48004%
Adequately Capitalized Amount	$ 466.8	$ 469.1
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	$ 583.5	$ 586.3
Well Capitalized Ratio	5.00%	5.00%