Regulatory Capital - Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) $ in Millions	Dec. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)
Total risk-based capital:
Actual amount	$ 2,976.5	$ 2,962.8
Capital to Risk Weighted Assets	0.1706	0.1438
Adequately capitalized Basel III phase-in schedule, amount	$ 1,396.1	$ 1,647.8
Adequately capitalized Basel III phase-in schedule, ratio (percent)	0.0800	0.0800
Adequately capitalized Basel III fully phased-in, amount	$ 1,832.3	$ 2,162.8
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	10.50%	10.50%
Well capitalized, amount	$ 1,745.1	$ 2,059.8
Capital Required to be Well Capitalized to Risk Weighted Assets	0.1000	0.1000
Tier 1 risk-based capital:
Actual amount	$ 2,508.8	$ 2,504.0
Tier One Risk Based Capital to Risk Weighted Assets	0.1438	0.1216
Adequately capitalized Basel III phase-in schedule, amount	$ 1,047.1	$ 1,235.9
Adequately capitalized Basel III phase-in schedule, ratio (percent)	0.0600	0.0600
Adequately capitalized Basel III fully phased-in, amount	$ 1,483.3	$ 1,750.8
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	8.50%	8.50%
Well capitalized, amount	$ 1,396.1	$ 1,647.8
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	0.0800	0.0800
Common Equity Tier One Capital [Abstract]
Actual amount	$ 2,508.8	$ 2,504.0
Common Equity Tier One Risk Based Capital To Risk Weighted Assets	14.38%	12.16%
Adequately capitalized Basel III phase-in schedule, amount	$ 785.3	$ 926.9
Adequately capitalized Basel III phase-in schedule, ratio (percent)	4.50%	4.50%
Adequately capitalized Basel III fully phased-in, amount	$ 1,221.6	$ 1,441.8
Tier One Common Equity Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	7.00%	7.00%
Well capitalized, amount	$ 1,134.3	$ 1,338.9
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets	6.50%	6.50%
Leverage capital ratio:
Actual amount	$ 2,508.8	$ 2,504.0
Tier One Leverage Capital to Average Assets	0.0961	0.0871
Adequately capitalized, amount	$ 1,043.7	$ 1,149.4
Adequately capitalized, ratio (percent)	0.0400	0.0400
Well capitalized, amount	$ 1,304.7	$ 1,436.8
Well capitalized, ratio (percent)	0.0500	0.0500
FIB
Total risk-based capital:
Actual amount	$ 2,656.6	$ 2,691.5
Capital to Risk Weighted Assets	0.1526	0.1310
Adequately capitalized Basel III phase-in schedule, amount	$ 1,392.3	$ 1,644.1
Adequately capitalized Basel III phase-in schedule, ratio (percent)	0.0800	0.0800
Adequately capitalized Basel III fully phased-in, amount	$ 1,827.4	$ 2,157.9
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	10.50%	10.50%
Well capitalized, amount	$ 1,740.4	$ 2,055.2
Capital Required to be Well Capitalized to Risk Weighted Assets	0.1000	0.1000
Tier 1 risk-based capital:
Actual amount	$ 2,458.8	$ 2,490.3
Tier One Risk Based Capital to Risk Weighted Assets	0.1413	0.1212
Adequately capitalized Basel III phase-in schedule, amount	$ 1,044.2	$ 1,233.1
Adequately capitalized Basel III phase-in schedule, ratio (percent)	0.0600	0.0600
Adequately capitalized Basel III fully phased-in, amount	$ 1,479.3	$ 1,746.9
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	8.50%	8.50%
Well capitalized, amount	$ 1,392.3	$ 1,644.1
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	0.0800	0.0800
Common Equity Tier One Capital [Abstract]
Actual amount	$ 2,458.8	$ 2,490.3
Common Equity Tier One Risk Based Capital To Risk Weighted Assets	14.13%	12.12%
Adequately capitalized Basel III phase-in schedule, amount	$ 783.2	$ 924.8
Adequately capitalized Basel III phase-in schedule, ratio (percent)	4.50%	4.50%
Adequately capitalized Basel III fully phased-in, amount	$ 1,218.3	$ 1,438.6
Tier One Common Equity Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	7.00%	7.00%
Well capitalized, amount	$ 1,131.3	$ 1,335.8
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets	6.50%	6.50%
Leverage capital ratio:
Actual amount	$ 2,458.8	$ 2,490.3
Tier One Leverage Capital to Average Assets	0.0944	0.0868
Adequately capitalized, amount	$ 1,041.6	$ 1,147.5
Adequately capitalized, ratio (percent)	0.0400	0.0400
Well capitalized, amount	$ 1,302.0	$ 1,434.4
Well capitalized, ratio (percent)	0.0500	0.0500