Regulatory Capital - Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Total risk-based capital:
Actual Amount	$ 1,131.5	$ 1,112.5
Actual Ratio	12.90412%	12.75962%
Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 865.9	$ 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	$ 920.7	$ 915.5
Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	10.50%	10.50%
Well Capitalized Amount	$ 876.8	$ 871.9
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	$ 1,058.0	$ 1,040.3
Actual Ratio	12.06583%	11.93154%
Tier One Risk Based Capital under Basel III Phase-in Schedule	$ 690.5	$ 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	$ 745.3	$ 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	$ 701.5	$ 697.5
Well Capitalized Ratio	8.00%	8.00%
Common equity tier one risk-based capital:
Actual Amount	$ 978.0	$ 962.4
Actual Ratio	11.15347%	11.03808%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 559.0	$ 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	$ 613.8	$ 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	$ 569.9	$ 566.7
Well Capitalized Ratio	6.50%	6.50%
Leverage capital ratio:
Actual Amount	$ 1,058.0	$ 1,040.3
Actual Ratio	9.06934%	8.85532%
Adequately Capitalized Amount	$ 466.6	$ 469.9
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	$ 583.3	$ 587.4
Well Capitalized Ratio	5.00%	5.00%
FIB
Total risk-based capital:
Actual Amount	$ 1,070.5	$ 1,066.6
Actual Ratio	12.24921%	12.29128%
Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 863.0	$ 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	$ 917.6	$ 911.2
Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	10.50%	10.50%
Well Capitalized Amount	$ 873.9	$ 867.8
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	$ 997.0	$ 994.4
Actual Ratio	11.4081%	11.45926%
Tier One Risk Based Capital under Basel III Phase-in Schedule	$ 688.2	$ 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	$ 742.8	$ 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	$ 699.1	$ 694.2
Well Capitalized Ratio	8.00%	8.00%
Common equity tier one risk-based capital:
Actual Amount	$ 997.0	$ 994.4
Actual Ratio	11.4081%	11.45926%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 557.1	$ 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	$ 611.7	$ 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	$ 568.0	$ 564.1
Well Capitalized Ratio	6.50%	6.50%
Leverage capital ratio:
Actual Amount	$ 997.0	$ 994.4
Actual Ratio	8.54894%	8.48004%
Adequately Capitalized Amount	$ 466.5	$ 469.1
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	$ 583.1	$ 586.3
Well Capitalized Ratio	5.00%	5.00%