Regulatory Capital (Details) $ in Millions	Mar. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained Earnings (Accumulated Deficit)	$ 1,168.6	$ 1,166.4
Total risk-based capital:
Actual, Amount	$ 2,981.3	$ 2,962.8
Actual, Ratio	0.1493	0.1438
Minimum Required for Capital Adequacy Purposes, Amount	$ 1,597.2	$ 1,647.8
Minimum Required for Capital Adequacy Purposes, Ratio	0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 2,096.3	$ 2,162.8
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	10.50%	10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 1,996.5	$ 2,059.8
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	$ 898.4	$ 926.9
Tier 1 risk-based capital:
Actual, Amount	$ 2,502.6	$ 2,504.0
Actual, Ratio	0.1253	0.1216
Minimum Required for Capital Adequacy Purposes, Amount	$ 1,197.9	$ 1,235.9
Minimum Required for Capital Adequacy Purposes, Ratio	0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,697.0	$ 1,750.8
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	8.50%	8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 1,597.2	$ 1,647.8
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	0.1253	0.1216
Banking Regulation, Common Equity Tier One Risk-Based Capital, Well Capitalized, Minimum	$ 1,297.7	$ 1,338.8
Banking Regulation, Common Equity Tier One Risk-Based Capital, Actual	2,502.6	2,504.0
Common equity tier 1 risk-based capital:
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,397.6	$ 1,441.8
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	7.00%	7.00%
Leverage capital ratio:
Actual, Amount	$ 2,502.6	$ 2,504.0
Actual, Ratio	0.0906	0.0871
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum	$ 1,104.6	$ 1,149.4
Capital Adequacy Purposes and Conservation Buffer, Ratio	0.0400	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 1,380.8	$ 1,436.8
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	0.0500	0.0500
FIB
Total risk-based capital:
Actual, Amount	$ 2,713.4	$ 2,691.5
Actual, Ratio	0.1363	0.1310
Minimum Required for Capital Adequacy Purposes, Amount	$ 1,593.2	$ 1,644.1
Minimum Required for Capital Adequacy Purposes, Ratio	0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 2,091.0	$ 2,157.9
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	10.50%	10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 1,991.5	$ 2,055.2
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	$ 896.2	$ 924.8
Tier 1 risk-based capital:
Actual, Amount	$ 2,492.2	$ 2,490.3
Actual, Ratio	0.1251	0.1212
Minimum Required for Capital Adequacy Purposes, Amount	$ 1,194.9	$ 1,233.1
Minimum Required for Capital Adequacy Purposes, Ratio	0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,692.7	$ 1,746.9
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	8.50%	8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 1,593.2	$ 1,644.1
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	0.1251	0.1212
Banking Regulation, Common Equity Tier One Risk-Based Capital, Well Capitalized, Minimum	$ 1,294.4	$ 1,335.8
Banking Regulation, Common Equity Tier One Risk-Based Capital, Actual	2,492.2	2,490.3
Common equity tier 1 risk-based capital:
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,394.0	$ 1,438.6
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	7.00%	7.00%
Leverage capital ratio:
Actual, Amount	$ 2,492.2	$ 2,490.3
Actual, Ratio	0.0904	0.0868
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum	$ 1,102.7	$ 1,147.5
Capital Adequacy Purposes and Conservation Buffer, Ratio	0.0400	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 1,378.4	$ 1,434.4
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	0.0500	0.0500