Regulatory Capital (Details) $ in Millions	Jun. 30, 2026 USD ($)	Dec. 31, 2025 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained Earnings (Accumulated Deficit)	$ 1,326.6	$ 1,274.2
Total risk-based capital:
Actual, Amount	$ 2,874.7	$ 2,976.5
Actual, Ratio	0.1730	0.1706
Minimum Required for Capital Adequacy Purposes, Amount	$ 1,329.1	$ 1,396.1
Minimum Required for Capital Adequacy Purposes, Ratio	0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,744.4	$ 1,832.3
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	10.50%	10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 1,661.4	$ 1,745.1
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	$ 747.6	$ 785.3
Tier 1 risk-based capital:
Actual, Amount	$ 2,415.6	$ 2,508.8
Actual, Ratio	0.1454	0.1438
Minimum Required for Capital Adequacy Purposes, Amount	$ 996.8	$ 1,047.1
Minimum Required for Capital Adequacy Purposes, Ratio	0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,412.2	$ 1,483.3
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	8.50%	8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 1,329.1	$ 1,396.1
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	0.1454	0.1438
Banking Regulation, Common Equity Tier One Risk-Based Capital, Well Capitalized, Minimum	$ 1,079.9	$ 1,134.3
Banking Regulation, Common Equity Tier One Risk-Based Capital, Actual	2,415.6	2,508.8
Common equity tier 1 risk-based capital:
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,163.0	$ 1,221.6
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	7.00%	7.00%
Leverage capital ratio:
Actual, Amount	$ 2,415.6	$ 2,508.8
Actual, Ratio	0.0959	0.0961
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum	$ 1,007.6	$ 1,043.7
Capital Adequacy Purposes and Conservation Buffer, Ratio	0.0400	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 1,259.5	$ 1,304.7
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	0.0500	0.0500
FIB
Total risk-based capital:
Actual, Amount	$ 2,558.6	$ 2,656.6
Actual, Ratio	0.1546	0.1526
Minimum Required for Capital Adequacy Purposes, Amount	$ 1,323.9	$ 1,392.3
Minimum Required for Capital Adequacy Purposes, Ratio	0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,737.6	$ 1,827.4
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	10.50%	10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 1,654.9	$ 1,740.4
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	$ 744.7	$ 783.2
Tier 1 risk-based capital:
Actual, Amount	$ 2,369.4	$ 2,458.8
Actual, Ratio	0.1432	0.1413
Minimum Required for Capital Adequacy Purposes, Amount	$ 992.9	$ 1,044.2
Minimum Required for Capital Adequacy Purposes, Ratio	0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,406.6	$ 1,479.3
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	8.50%	8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 1,323.9	$ 1,392.3
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	0.1432	0.1413
Banking Regulation, Common Equity Tier One Risk-Based Capital, Well Capitalized, Minimum	$ 1,075.7	$ 1,131.3
Banking Regulation, Common Equity Tier One Risk-Based Capital, Actual	2,369.4	2,458.8
Common equity tier 1 risk-based capital:
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,158.4	$ 1,218.3
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	7.00%	7.00%
Leverage capital ratio:
Actual, Amount	$ 2,369.4	$ 2,458.8
Actual, Ratio	0.0942	0.0944
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum	$ 1,005.7	$ 1,041.6
Capital Adequacy Purposes and Conservation Buffer, Ratio	0.0400	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount	$ 1,257.1	$ 1,302.0
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	0.0500	0.0500