Regulatory Capital (Details) $ in Millions	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained Earnings (Accumulated Deficit)	$ 988.2	$ 962.1
Total risk-based capital:
Actual, Amount	$ 1,597.3	$ 1,575.7
Actual, Ratio	0.1404	0.1419
Minimum Required for Capital Adequacy Purposes, Amount	$ 910.1	$ 888.3
Minimum Required for Capital Adequacy Purposes, Ratio	0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,194.4	$ 1,165.8
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	10.50%	10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 1,137.6	$ 1,110.3
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	0.1000	0.1000
Tier 1 risk-based capital:
Actual, Amount	$ 1,396.4	$ 1,369.0
Actual, Ratio	0.1228	0.1233
Minimum Required for Capital Adequacy Purposes, Amount	$ 682.5	$ 666.2
Minimum Required for Capital Adequacy Purposes, Ratio	0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 966.9	$ 943.8
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	8.50%	8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 910.1	$ 888.3
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	0.0800	0.0800
Common equity tier 1 risk-based capital:
Actual, Amount	$ 1,309.4	$ 1,284.9
Actual, Ratio	11.51%	11.57%
Minimum Required for Capital Adequacy Purposes, Amount	$ 511.9	$ 499.6
Minimum Required for Capital Adequacy Purposes, Ratio	4.50%	4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 796.3	$ 777.2
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	7.00%	7.00%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 739.4	$ 721.7
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	6.50%	6.50%
Leverage capital ratio:
Actual, Amount	$ 1,396.4	$ 1,369.0
Actual, Ratio	0.0812	0.0816
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum	$ 687.7	$ 671.0
Capital Adequacy Purposes and Conservation Buffer, Ratio	0.0400	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 859.6	$ 838.7
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	0.0500	0.0500
FIB
Total risk-based capital:
Actual, Amount	$ 1,437.1	$ 1,426.8
Actual, Ratio	0.1267	0.1289
Minimum Required for Capital Adequacy Purposes, Amount	$ 907.4	$ 885.6
Minimum Required for Capital Adequacy Purposes, Ratio	0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,191.0	$ 1,162.3
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	10.50%	10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 1,134.3	$ 1,107.0
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	0.1000	0.1000
Tier 1 risk-based capital:
Actual, Amount	$ 1,336.3	$ 1,320.1
Actual, Ratio	0.1178	0.1193
Minimum Required for Capital Adequacy Purposes, Amount	$ 680.6	$ 664.2
Minimum Required for Capital Adequacy Purposes, Ratio	0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 964.1	$ 940.9
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	8.50%	8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 907.4	$ 885.6
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	0.0800	0.0800
Common equity tier 1 risk-based capital:
Actual, Amount	$ 1,336.3	$ 1,320.1
Actual, Ratio	11.78%	11.93%
Minimum Required for Capital Adequacy Purposes, Amount	$ 510.4	$ 498.1
Minimum Required for Capital Adequacy Purposes, Ratio	4.50%	4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 794.0	$ 774.9
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	7.00%	7.00%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 737.3	$ 719.5
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	6.50%	6.50%
Leverage capital ratio:
Actual, Amount	$ 1,336.3	$ 1,320.1
Actual, Ratio	0.0775	0.0788
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum	$ 689.3	$ 669.7
Capital Adequacy Purposes and Conservation Buffer, Ratio	0.0400	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 861.6	$ 837.2
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	0.0500	0.0500