Regulatory Capital (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Total risk-based capital:
Actual, Amount	$ 1,460.2	$ 1,285.0
Actual, Ratio	13.71%	12.99%
Minimum Required for Capital Adequacy Purposes, Amount	$ 851.9	$ 791.2
Minimum Required for Capital Adequacy Purposes, Ratio	8.00%	8.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,118.1	$ 976.6
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	10.50%	9.875%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 1,064.9	$ 989.0
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual, Amount	$ 1,385.2	$ 1,212.0
Actual, Ratio	13.01%	12.26%
Minimum Required for Capital Adequacy Purposes, Amount	$ 638.9	$ 593.4
Minimum Required for Capital Adequacy Purposes, Ratio	6.00%	6.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 905.2	$ 778.8
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	8.50%	7.875%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 851.9	$ 791.2
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	8.00%	8.00%
Common equity tier 1 risk-based capital:
Actual, Amount	$ 1,298.3	$ 1,127.8
Actual, Ratio	12.19%	11.40%
Minimum Required for Capital Adequacy Purposes, Amount	$ 479.2	$ 445.0
Minimum Required for Capital Adequacy Purposes, Ratio	4.50%	4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 745.4	$ 630.5
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	7.00%	6.375%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 692.2	$ 642.8
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	6.50%	6.50%
Leverage capital ratio:
Actual, Amount	$ 1,385.2	$ 1,212.0
Actual, Ratio	9.96%	9.47%
Capital Adequacy Purposes and Conservation Buffer, Amount	$ 556.2	$ 511.9
Capital Adequacy Purposes and Conservation Buffer, Ratio	4.00%	4.00%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 695.3	$ 639.9
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	5.00%	5.00%
FIB
Total risk-based capital:
Actual, Amount	$ 1,308.6	$ 1,184.5
Actual, Ratio	12.33%	12.01%
Minimum Required for Capital Adequacy Purposes, Amount	$ 849.0	$ 788.8
Minimum Required for Capital Adequacy Purposes, Ratio	8.00%	8.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,114.3	$ 973.7
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	10.50%	9.875%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 1,061.3	$ 986.0
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual, Amount	$ 1,233.7	$ 1,111.6
Actual, Ratio	11.62%	11.27%
Minimum Required for Capital Adequacy Purposes, Amount	$ 636.8	$ 591.6
Minimum Required for Capital Adequacy Purposes, Ratio	6.00%	6.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 902.1	$ 776.5
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	8.50%	7.875%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 849.0	$ 788.8
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	8.00%	8.00%
Common equity tier 1 risk-based capital:
Actual, Amount	$ 1,233.7	$ 1,111.6
Actual, Ratio	11.62%	11.27%
Minimum Required for Capital Adequacy Purposes, Amount	$ 477.6	$ 443.7
Minimum Required for Capital Adequacy Purposes, Ratio	4.50%	4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 742.9	$ 628.6
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	7.00%	6.375%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 689.8	$ 640.9
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	6.50%	6.50%
Leverage capital ratio:
Actual, Amount	$ 1,233.7	$ 1,111.6
Actual, Ratio	8.96%	8.97%
Capital Adequacy Purposes and Conservation Buffer, Amount	$ 550.5	$ 495.9
Capital Adequacy Purposes and Conservation Buffer, Ratio	4.00%	4.00%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 688.1	$ 619.8
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	5.00%	5.00%