Regulatory Capital (Details) $ in Millions	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained Earnings (Accumulated Deficit)	$ 912.5			$ 953.6
Total risk-based capital:
Actual, Amount	$ 1,600.1			$ 1,495.3
Actual, Ratio	0.1476			0.1410
Minimum Required for Capital Adequacy Purposes, Amount	$ 867.6			$ 848.5
Minimum Required for Capital Adequacy Purposes, Ratio	0.08000			0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,138.7			$ 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	$ 1,084.4			$ 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	$ 1,393.1			$ 1,422.3
Actual, Ratio	0.1285			0.1341
Minimum Required for Capital Adequacy Purposes, Amount	$ 650.7			$ 636.3
Minimum Required for Capital Adequacy Purposes, Ratio	0.06000			0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 921.8			$ 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	$ 867.6			$ 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	$ 1,309.0			$ 1,338.2
Actual, Ratio	12.07%			12.62%
Minimum Required for Capital Adequacy Purposes, Amount	$ 488.0			$ 477.3
Minimum Required for Capital Adequacy Purposes, Ratio	4.50%			4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 759.1			$ 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	$ 704.9			$ 689.4
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	6.50%			6.50%
Leverage capital ratio:
Actual, Amount	$ 1,393.1			$ 1,422.3
Actual, Ratio	0.0922			0.1013
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum	$ 604.6			$ 561.6
Capital Adequacy Purposes and Conservation Buffer, Ratio	0.04000			0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 755.8			$ 702.0
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	0.0500			0.0500
FIB
Total risk-based capital:
Actual, Amount	$ 1,404.5			$ 1,321.4
Actual, Ratio	0.1299			0.1250
Minimum Required for Capital Adequacy Purposes, Amount	$ 865.0			$ 845.8
Minimum Required for Capital Adequacy Purposes, Ratio	0.08000			0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,135.3			$ 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	$ 1,081.2			$ 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	$ 1,297.4			$ 1,248.4
Actual, Ratio	0.1200			0.1181
Minimum Required for Capital Adequacy Purposes, Amount	$ 648.7			$ 634.3
Minimum Required for Capital Adequacy Purposes, Ratio	0.06000			0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 919.0			$ 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	$ 865.0			$ 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	$ 1,297.4			$ 1,248.4
Actual, Ratio	12.00%			11.81%
Minimum Required for Capital Adequacy Purposes, Amount	$ 486.5			$ 475.7
Minimum Required for Capital Adequacy Purposes, Ratio	4.50%			4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 756.8			$ 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	$ 702.8			$ 687.2
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	6.50%			6.50%
Leverage capital ratio:
Actual, Amount	$ 1,297.4			$ 1,248.4
Actual, Ratio	0.0860			0.0891
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum	$ 603.4			$ 560.4
Capital Adequacy Purposes and Conservation Buffer, Ratio	0.04000			0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 754.3			$ 700.4
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	0.0500			0.0500
Accounting Standards Update 2016-13 [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained Earnings (Accumulated Deficit)		$ (24.1)
Cumulative Effect, Period of Adoption, Adjustment
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained Earnings (Accumulated Deficit)			$ (24.1)