Regulatory Capital (Details) $ in Millions	Sep. 30, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained Earnings (Accumulated Deficit)	$ 938.9		$ 953.6
Total risk-based capital:
Actual, Amount	$ 1,585.8		$ 1,495.3
Actual, Ratio	0.1445		0.1410
Minimum Required for Capital Adequacy Purposes, Amount	$ 877.7		$ 848.5
Minimum Required for Capital Adequacy Purposes, Ratio	0.0800		0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,152.0		$ 1,113.6
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	10.50%		10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 1,097.1		$ 1,060.6
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	0.1000		0.1000
Tier 1 risk-based capital:
Actual, Amount	$ 1,378.3		$ 1,422.3
Actual, Ratio	0.1256		0.1341
Minimum Required for Capital Adequacy Purposes, Amount	$ 658.3		$ 636.3
Minimum Required for Capital Adequacy Purposes, Ratio	0.0600		0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 932.6		$ 901.5
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	8.50%		8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 877.7		$ 848.5
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	0.0800		0.0800
Common equity tier 1 risk-based capital:
Actual, Amount	$ 1,294.2		$ 1,338.2
Actual, Ratio	11.80%		12.62%
Minimum Required for Capital Adequacy Purposes, Amount	$ 493.7		$ 477.3
Minimum Required for Capital Adequacy Purposes, Ratio	4.50%		4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 768.0		$ 742.4
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	7.00%		7.00%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 713.1		$ 689.4
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	6.50%		6.50%
Leverage capital ratio:
Actual, Amount	$ 1,378.3		$ 1,422.3
Actual, Ratio	0.0862		0.1013
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum	$ 639.3		$ 561.6
Capital Adequacy Purposes and Conservation Buffer, Ratio	0.0400		0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 799.1		$ 702.0
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	0.0500		0.0500
FIB
Total risk-based capital:
Actual, Amount	$ 1,412.5		$ 1,321.4
Actual, Ratio	0.1291		0.1250
Minimum Required for Capital Adequacy Purposes, Amount	$ 875.1		$ 845.8
Minimum Required for Capital Adequacy Purposes, Ratio	0.0800		0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,148.6		$ 1,110.1
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	10.50%		10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 1,093.9		$ 1,057.2
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	0.1000		0.1000
Tier 1 risk-based capital:
Actual, Amount	$ 1,305.1		$ 1,248.4
Actual, Ratio	0.1193		0.1181
Minimum Required for Capital Adequacy Purposes, Amount	$ 656.3		$ 634.3
Minimum Required for Capital Adequacy Purposes, Ratio	0.0600		0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 929.8		$ 898.6
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	8.50%		8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 875.1		$ 845.8
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	0.0800		0.0800
Common equity tier 1 risk-based capital:
Actual, Amount	$ 1,305.1		$ 1,248.4
Actual, Ratio	11.93%		11.81%
Minimum Required for Capital Adequacy Purposes, Amount	$ 492.3		$ 475.7
Minimum Required for Capital Adequacy Purposes, Ratio	4.50%		4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 765.7		$ 740.0
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	7.00%		7.00%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 711.0		$ 687.2
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	6.50%		6.50%
Leverage capital ratio:
Actual, Amount	$ 1,305.1		$ 1,248.4
Actual, Ratio	0.0818		0.0891
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum	$ 638.2		$ 560.4
Capital Adequacy Purposes and Conservation Buffer, Ratio	0.0400		0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 797.8		$ 700.4
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	0.0500		0.0500
Cumulative Effect, Period of Adoption, Adjustment
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained Earnings (Accumulated Deficit)		$ (24.1)