Regulatory Capital (Details) $ in Millions	Jan. 01, 2020 USD ($)	Sep. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained Earnings (Accumulated Deficit)		$ 1,122.3	$ 1,072.7
Total risk-based capital:
Actual, Amount		$ 2,957.6	$ 2,875.8
Actual, Ratio		0.1319	0.1248
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,794.5	$ 1,843.2
Minimum Required for Capital Adequacy Purposes, Ratio		0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 2,355.3	$ 2,419.2
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		10.50%	10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 2,243.1	$ 2,304.0
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		$ 1,009.4	$ 1,036.8
Tier 1 risk-based capital:
Actual, Amount		$ 2,471.6	$ 2,408.8
Actual, Ratio		0.1102	0.1045
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,345.9	$ 1,382.4
Minimum Required for Capital Adequacy Purposes, Ratio		0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,906.6	$ 1,958.4
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		8.50%	8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,794.5	$ 1,843.2
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		0.1102	0.1045
Banking Regulation, Common Equity Tier One Risk-Based Capital, Well Capitalized, Minimum		$ 1,458.0	$ 1,497.6
Banking Regulation, Common Equity Tier One Risk-Based Capital, Actual		2,471.6	2,408.8
Common equity tier 1 risk-based capital:
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,570.2	$ 1,612.8
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		7.00%	7.00%
Leverage capital ratio:
Actual, Amount		$ 2,471.6	$ 2,408.8
Actual, Ratio		0.0822	0.0775
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum		$ 1,202.8	$ 1,242.9
Capital Adequacy Purposes and Conservation Buffer, Ratio		0.0400	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,503.5	$ 1,553.6
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		$ 2,782.1	$ 2,713.5
Actual, Ratio		0.1243	0.1180
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,790.8	$ 1,839.6
Minimum Required for Capital Adequacy Purposes, Ratio		0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 2,350.5	$ 2,414.5
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		10.50%	10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 2,238.6	$ 2,299.5
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		$ 1,007.3	$ 1,034.8
Tier 1 risk-based capital:
Actual, Amount		$ 2,553.7	$ 2,504.1
Actual, Ratio		0.1141	0.1089
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,343.1	$ 1,379.7
Minimum Required for Capital Adequacy Purposes, Ratio		0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,902.8	$ 1,954.6
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		8.50%	8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,790.8	$ 1,839.6
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		0.1141	0.1089
Banking Regulation, Common Equity Tier One Risk-Based Capital, Well Capitalized, Minimum		$ 1,455.1	$ 1,494.7
Banking Regulation, Common Equity Tier One Risk-Based Capital, Actual		2,553.7	2,504.1
Common equity tier 1 risk-based capital:
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,567.0	$ 1,609.7
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		7.00%	7.00%
Leverage capital ratio:
Actual, Amount		$ 2,553.7	$ 2,504.1
Actual, Ratio		0.0850	0.0807
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum		$ 1,201.1	$ 1,241.1
Capital Adequacy Purposes and Conservation Buffer, Ratio		0.0400	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,501.3	$ 1,551.4
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0500	0.0500