Regulatory Capital (Details) $ in Millions	Jan. 01, 2020 USD ($)	Sep. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained Earnings (Accumulated Deficit)		$ 1,035.8	$ 1,052.6
Total risk-based capital:
Actual, Amount		$ 2,820.2	$ 1,659.3
Actual, Ratio		0.1250	0.1411
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,804.7	$ 940.9
Minimum Required for Capital Adequacy Purposes, Ratio		0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 2,368.7	$ 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		$ 2,255.9	$ 1,176.2
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.1000	0.1000
Banking Regulation, Common Equity Tier One Risk-Based Capital, Capital Adequacy, Minimum		$ 1,015.1	$ 529.3
Tier 1 risk-based capital:
Actual, Amount		$ 2,366.8	$ 1,469.0
Actual, Ratio		0.1049	0.1249
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,353.5	$ 705.7
Minimum Required for Capital Adequacy Purposes, Ratio		0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,917.5	$ 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		$ 1,804.7	$ 940.9
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0800	0.0800
Banking Regulation, Common Equity Tier One Risk-Based Capital Ratio, Well Capitalized, Minimum		0.0650	0.0650
Banking Regulation, Common Equity Tier One Risk-Based Capital Ratio, Capital Adequacy, Minimum		0.0450	0.0450
Banking Regulation, Common Equity Tier One Risk-Based Capital Ratio, Actual		0.1049	0.1177
Banking Regulation, Common Equity Tier One Risk-Based Capital, Well Capitalized, Minimum		$ 1,466.3	$ 764.5
Banking Regulation, Common Equity Tier One Risk-Based Capital, Actual		2,366.8	1,384.8
Common equity tier 1 risk-based capital:
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,579.1	$ 823.3
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		7.00%	7.00%
Leverage capital ratio:
Actual, Amount		$ 2,366.8	$ 1,469.0
Actual, Ratio		0.0767	0.0768
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum		$ 1,234.4	$ 765.5
Capital Adequacy Purposes and Conservation Buffer, Ratio		0.0400	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,543.0	$ 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		$ 2,704.8	$ 1,472.5
Actual, Ratio		0.1201	0.1256
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,801.1	$ 938.0
Minimum Required for Capital Adequacy Purposes, Ratio		0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 2,363.9	$ 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		$ 2,251.4	$ 1,172.5
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.1000	0.1000
Banking Regulation, Common Equity Tier One Risk-Based Capital, Capital Adequacy, Minimum		$ 1,013.1	$ 527.6
Tier 1 risk-based capital:
Actual, Amount		$ 2,508.9	$ 1,382.2
Actual, Ratio		0.1114	0.1179
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,350.8	$ 703.5
Minimum Required for Capital Adequacy Purposes, Ratio		0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,913.7	$ 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		$ 1,801.1	$ 938.0
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0800	0.0800
Banking Regulation, Common Equity Tier One Risk-Based Capital Ratio, Well Capitalized, Minimum		0.0650	0.0650
Banking Regulation, Common Equity Tier One Risk-Based Capital Ratio, Capital Adequacy, Minimum		0.0450	0.0450
Banking Regulation, Common Equity Tier One Risk-Based Capital Ratio, Actual		0.1114	0.1179
Banking Regulation, Common Equity Tier One Risk-Based Capital, Well Capitalized, Minimum		$ 1,463.4	$ 762.1
Banking Regulation, Common Equity Tier One Risk-Based Capital, Actual		2,508.9	1,382.2
Common equity tier 1 risk-based capital:
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,576.0	$ 820.8
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		7.00%	7.00%
Leverage capital ratio:
Actual, Amount		$ 2,508.9	$ 1,382.2
Actual, Ratio		0.0814	0.0724
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum		$ 1,232.2	$ 764.1
Capital Adequacy Purposes and Conservation Buffer, Ratio		0.0400	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,540.2	$ 955.1
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0500	0.0500