Regulatory Capital (Details) $ in Millions	Jan. 01, 2020 USD ($)	Jun. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained Earnings (Accumulated Deficit)		$ 993.8	$ 1,052.6
Total risk-based capital:
Actual, Amount		$ 2,908.7	$ 1,659.3
Actual, Ratio		0.1360	0.1411
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,710.7	$ 940.9
Minimum Required for Capital Adequacy Purposes, Ratio		0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 2,245.2	$ 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		$ 2,138.3	$ 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		$ 962.2	$ 529.3
Tier 1 risk-based capital:
Actual, Amount		$ 2,451.4	$ 1,469.0
Actual, Ratio		0.1146	0.1249
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,283.0	$ 705.7
Minimum Required for Capital Adequacy Purposes, Ratio		0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,817.6	$ 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		$ 1,710.7	$ 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		0.1146	0.1177
Banking Regulation, Common Equity Tier One Risk-Based Capital, Well Capitalized, Minimum		$ 1,389.9	$ 764.5
Banking Regulation, Common Equity Tier One Risk-Based Capital, Actual		2,451.4	1,384.8
Common equity tier 1 risk-based capital:
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,496.8	$ 823.3
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		7.00%	7.00%
Leverage capital ratio:
Actual, Amount		$ 2,451.4	$ 1,469.0
Actual, Ratio		0.0772	0.0768
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum		$ 1,269.5	$ 765.5
Capital Adequacy Purposes and Conservation Buffer, Ratio		0.0400	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,586.8	$ 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		$ 2,743.7	$ 1,472.5
Actual, Ratio		0.1287	0.1256
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,705.5	$ 938.0
Minimum Required for Capital Adequacy Purposes, Ratio		0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 2,238.5	$ 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		$ 2,131.9	$ 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		$ 959.3	$ 527.6
Tier 1 risk-based capital:
Actual, Amount		$ 2,544.0	$ 1,382.2
Actual, Ratio		0.1193	0.1179
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,279.1	$ 703.5
Minimum Required for Capital Adequacy Purposes, Ratio		0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,812.1	$ 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		$ 1,705.5	$ 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		0.1193	0.1179
Banking Regulation, Common Equity Tier One Risk-Based Capital, Well Capitalized, Minimum		$ 1,385.7	$ 762.1
Banking Regulation, Common Equity Tier One Risk-Based Capital, Actual		2,544.0	1,382.2
Common equity tier 1 risk-based capital:
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,492.3	$ 820.8
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		7.00%	7.00%
Leverage capital ratio:
Actual, Amount		$ 2,544.0	$ 1,382.2
Actual, Ratio		0.0803	0.0724
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum		$ 1,267.0	$ 764.1
Capital Adequacy Purposes and Conservation Buffer, Ratio		0.0400	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,583.7	$ 955.1
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0500	0.0500