Regulatory Capital (Details) $ in Millions	Mar. 31, 2026 USD ($)	Dec. 31, 2025 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained Earnings (Accumulated Deficit)	$ 1,288.7	$ 1,274.2
Total risk-based capital:
Actual, Amount	$ 2,912.7	$ 2,976.5
Actual, Ratio	0.1707	0.1706
Minimum Required for Capital Adequacy Purposes, Amount	$ 1,365.4	$ 1,396.1
Minimum Required for Capital Adequacy Purposes, Ratio	0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,792.1	$ 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	$ 1,706.8	$ 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	$ 768.0	$ 785.3
Tier 1 risk-based capital:
Actual, Amount	$ 2,440.8	$ 2,508.8
Actual, Ratio	0.1430	0.1438
Minimum Required for Capital Adequacy Purposes, Amount	$ 1,024.1	$ 1,047.1
Minimum Required for Capital Adequacy Purposes, Ratio	0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,450.8	$ 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	$ 1,365.4	$ 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	0.1430	0.1438
Banking Regulation, Common Equity Tier One Risk-Based Capital, Well Capitalized, Minimum	$ 1,109.4	$ 1,134.3
Banking Regulation, Common Equity Tier One Risk-Based Capital, Actual	2,440.8	2,508.8
Common equity tier 1 risk-based capital:
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,194.7	$ 1,221.6
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	7.00%	7.00%
Leverage capital ratio:
Actual, Amount	$ 2,440.8	$ 2,508.8
Actual, Ratio	0.0956	0.0961
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum	$ 1,021.3	$ 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	$ 1,276.6	$ 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	$ 2,577.7	$ 2,656.6
Actual, Ratio	0.1514	0.1526
Minimum Required for Capital Adequacy Purposes, Amount	$ 1,362.2	$ 1,392.3
Minimum Required for Capital Adequacy Purposes, Ratio	0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,787.9	$ 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	$ 1,702.7	$ 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	$ 766.2	$ 783.2
Tier 1 risk-based capital:
Actual, Amount	$ 2,375.6	$ 2,458.8
Actual, Ratio	0.1395	0.1413
Minimum Required for Capital Adequacy Purposes, Amount	$ 1,021.6	$ 1,044.2
Minimum Required for Capital Adequacy Purposes, Ratio	0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,447.3	$ 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	$ 1,362.2	$ 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	0.1395	0.1413
Banking Regulation, Common Equity Tier One Risk-Based Capital, Well Capitalized, Minimum	$ 1,106.8	$ 1,131.3
Banking Regulation, Common Equity Tier One Risk-Based Capital, Actual	2,375.6	2,458.8
Common equity tier 1 risk-based capital:
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount	$ 1,191.9	$ 1,218.3
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio	7.00%	7.00%
Leverage capital ratio:
Actual, Amount	$ 2,375.6	$ 2,458.8
Actual, Ratio	0.0932	0.0944
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum	$ 1,019.1	$ 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	$ 1,273.9	$ 1,302.0
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio	0.0500	0.0500