Regulatory Capital (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Total risk-based capital:
Actual, Amount	$ 1,420.5	$ 1,285.0
Actual, Ratio	13.43%	12.99%
Minimum Required for Capital Adequacy Purposes, Amount	$ 846.0	$ 791.2
Minimum Required for Capital Adequacy Purposes, Ratio	8.00%	8.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,110.4	$ 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	$ 1,057.6	$ 989.0
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual, Amount	$ 1,346.3	$ 1,212.0
Actual, Ratio	12.73%	12.26%
Minimum Required for Capital Adequacy Purposes, Amount	$ 634.5	$ 593.4
Minimum Required for Capital Adequacy Purposes, Ratio	6.00%	6.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 898.9	$ 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	$ 846.0	$ 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	$ 1,262.3	$ 1,127.8
Actual, Ratio	11.94%	11.40%
Minimum Required for Capital Adequacy Purposes, Amount	$ 475.9	$ 445.0
Minimum Required for Capital Adequacy Purposes, Ratio	4.50%	4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 740.3	$ 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	$ 687.4	$ 642.8
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	6.50%	6.50%
Leverage capital ratio:
Actual, Amount	$ 1,346.3	$ 1,212.0
Actual, Ratio	9.94%	9.47%
Capital Adequacy Purposes and Conservation Buffer, Amount	$ 541.7	$ 511.9
Capital Adequacy Purposes and Conservation Buffer, Ratio	4.00%	4.00%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 677.2	$ 639.9
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	5.00%	5.00%
FIB
Total risk-based capital:
Actual, Amount	$ 1,294.6	$ 1,184.5
Actual, Ratio	12.27%	12.01%
Minimum Required for Capital Adequacy Purposes, Amount	$ 844.3	$ 788.8
Minimum Required for Capital Adequacy Purposes, Ratio	8.00%	8.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,108.1	$ 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	$ 1,055.4	$ 986.0
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual, Amount	$ 1,220.4	$ 1,111.6
Actual, Ratio	11.56%	11.27%
Minimum Required for Capital Adequacy Purposes, Amount	$ 633.2	$ 591.6
Minimum Required for Capital Adequacy Purposes, Ratio	6.00%	6.00%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 897.1	$ 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	$ 844.3	$ 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	$ 1,220.4	$ 1,111.6
Actual, Ratio	11.56%	11.27%
Minimum Required for Capital Adequacy Purposes, Amount	$ 474.9	$ 443.7
Minimum Required for Capital Adequacy Purposes, Ratio	4.50%	4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 738.8	$ 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	$ 686.0	$ 640.9
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	6.50%	6.50%
Leverage capital ratio:
Actual, Amount	$ 1,220.4	$ 1,111.6
Actual, Ratio	9.01%	8.97%
Capital Adequacy Purposes and Conservation Buffer, Amount	$ 541.9	$ 495.9
Capital Adequacy Purposes and Conservation Buffer, Ratio	4.00%	4.00%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 677.4	$ 619.8
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	5.00%	5.00%