Regulatory Capital (Details) $ in Millions	Jan. 01, 2020 USD ($)	Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained Earnings (Accumulated Deficit)		$ 974.5	$ 1,052.6
Total risk-based capital:
Actual, Amount		$ 2,930.9	$ 1,659.3
Actual, Ratio		0.1427	0.1411
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,643.5	$ 940.9
Minimum Required for Capital Adequacy Purposes, Ratio		0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 2,157.1	$ 1,235.0
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		10.50%	10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 2,054.4	$ 1,176.2
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.1000	0.1000
Tier 1 risk-based capital:
Actual, Amount		$ 2,446.7	$ 1,469.0
Actual, Ratio		0.1191	0.1249
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,232.6	$ 705.7
Minimum Required for Capital Adequacy Purposes, Ratio		0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,746.2	$ 999.7
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		8.50%	8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,643.5	$ 940.9
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0800	0.0800
Common equity tier 1 risk-based capital:
Actual, Amount		$ 2,446.7	$ 1,384.8
Actual, Ratio		11.91%	11.77%
Minimum Required for Capital Adequacy Purposes, Amount		$ 924.5	$ 529.3
Minimum Required for Capital Adequacy Purposes, Ratio		4.50%	4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,438.1	$ 823.3
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		7.00%	7.00%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,335.3	$ 764.5
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		6.50%	6.50%
Leverage capital ratio:
Actual, Amount		$ 2,446.7	$ 1,469.0
Actual, Ratio		0.0896	0.0768
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum		$ 1,091.7	$ 765.5
Capital Adequacy Purposes and Conservation Buffer, Ratio		0.0400	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,364.6	$ 956.9
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0500	0.0500
Income Tax Effects Allocated Directly to Equity, Cumulative Effect of Change in Accounting Principle	$ 24.1
FIB
Total risk-based capital:
Actual, Amount		$ 2,787.3	$ 1,472.5
Actual, Ratio		0.1361	0.1256
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,638.3	$ 938.0
Minimum Required for Capital Adequacy Purposes, Ratio		0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 2,150.2	$ 1,231.1
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		10.50%	10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 2,047.9	$ 1,172.5
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.1000	0.1000
Tier 1 risk-based capital:
Actual, Amount		$ 2,560.7	$ 1,382.2
Actual, Ratio		0.1250	0.1179
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,228.7	$ 703.5
Minimum Required for Capital Adequacy Purposes, Ratio		0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,740.7	$ 996.6
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		8.50%	8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,638.3	$ 938.0
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0800	0.0800
Common equity tier 1 risk-based capital:
Actual, Amount		$ 2,560.7	$ 1,382.2
Actual, Ratio		12.50%	11.79%
Minimum Required for Capital Adequacy Purposes, Amount		$ 921.5	$ 527.6
Minimum Required for Capital Adequacy Purposes, Ratio		4.50%	4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,433.5	$ 820.8
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		7.00%	7.00%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,331.1	$ 762.1
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		6.50%	6.50%
Leverage capital ratio:
Actual, Amount		$ 2,560.7	$ 1,382.2
Actual, Ratio		0.0936	0.0724
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum		$ 1,093.8	$ 764.1
Capital Adequacy Purposes and Conservation Buffer, Ratio		0.0400	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,367.2	$ 955.1
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0500	0.0500