Regulatory Capital - Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Total risk-based capital:
Actual Amount	$ 1,310.3	$ 1,285.0
Actual Ratio	13.23%	12.99%
Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 1,040.0	$ 976.6
Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	10.50%	9.875%
Well Capitalized Amount	$ 990.5	$ 989.0
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	$ 1,237.9	$ 1,212.0
Actual Ratio	12.50%	12.26%
Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 841.9	$ 778.8
Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	8.50%	7.875%
Well Capitalized Amount	$ 792.4	$ 791.2
Well Capitalized Ratio	8.00%	8.00%
Common equity tier one risk-based capital:
Actual Amount	$ 1,153.7	$ 1,127.8
Actual Ratio	11.65%	11.40%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 445.7	$ 445.0
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	4.50%	4.50%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 693.4	$ 630.5
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	7.00%	6.375%
Well Capitalized Amount	$ 643.8	$ 642.8
Well Capitalized Ratio	6.50%	6.50%
Leverage capital ratio:
Actual Amount	$ 1,237.9	$ 1,212.0
Actual Ratio	9.76%	9.47%
Adequately Capitalized Amount	$ 507.5	$ 511.9
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	$ 634.4	$ 639.9
Well Capitalized Ratio	5.00%	5.00%
Capital Required for Capital Adequacy	$ 792.4	$ 791.2
Capital Required for Capital Adequacy to Risk Weighted Assets	8.00%	8.00%
Tier One Risk Based Capital Required for Capital Adequacy	$ 594.3	$ 593.4
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets	6.00%	6.00%
FIB
Total risk-based capital:
Actual Amount	$ 1,201.4	$ 1,184.5
Actual Ratio	12.16%	12.01%
Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 1,037.1	$ 973.7
Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	10.50%	9.875%
Well Capitalized Amount	$ 987.8	$ 986.0
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	$ 1,129.0	$ 1,111.6
Actual Ratio	11.43%	11.27%
Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 839.6	$ 776.5
Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	8.50%	7.875%
Well Capitalized Amount	$ 790.2	$ 788.8
Well Capitalized Ratio	8.00%	8.00%
Common equity tier one risk-based capital:
Actual Amount	$ 1,129.0	$ 1,111.6
Actual Ratio	11.43%	11.27%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 444.5	$ 443.7
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	4.50%	4.50%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 691.4	$ 628.6
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	7.00%	6.375%
Well Capitalized Amount	$ 642.0	$ 640.9
Well Capitalized Ratio	6.50%	6.50%
Leverage capital ratio:
Actual Amount	$ 1,129.0	$ 1,111.6
Actual Ratio	8.92%	8.97%
Adequately Capitalized Amount	$ 506.3	$ 495.9
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	$ 632.8	$ 619.8
Well Capitalized Ratio	5.00%	5.00%
Capital Required for Capital Adequacy	$ 790.2	$ 788.8
Capital Required for Capital Adequacy to Risk Weighted Assets	8.00%	8.00%
Tier One Risk Based Capital Required for Capital Adequacy	$ 592.7	$ 591.6
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets	6.00%	6.00%