Regulatory Capital - Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) $ in Millions	Jan. 01, 2020 USD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Total risk-based capital:
Actual amount		$ 2,962.8	$ 2,941.1
Capital to Risk Weighted Assets		0.1438	0.1328
Adequately capitalized Basel III phase-in schedule, amount		$ 1,647.8	$ 1,771.6
Adequately capitalized Basel III phase-in schedule, ratio (percent)		0.0800	0.0800
Adequately capitalized Basel III fully phased-in, amount		$ 2,162.8	$ 2,325.2
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets		10.50%	10.50%
Well capitalized, amount		$ 2,059.8	$ 2,214.5
Capital Required to be Well Capitalized to Risk Weighted Assets		0.1000	0.1000
Tier 1 risk-based capital:
Actual amount		$ 2,504.0	$ 2,454.4
Tier One Risk Based Capital to Risk Weighted Assets		0.1216	0.1108
Adequately capitalized Basel III phase-in schedule, amount		$ 1,235.9	$ 1,328.7
Adequately capitalized Basel III phase-in schedule, ratio (percent)		0.0600	0.0600
Adequately capitalized Basel III fully phased-in, amount		$ 1,750.8	$ 1,882.3
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets		8.50%	8.50%
Well capitalized, amount		$ 1,647.8	$ 1,771.6
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		$ 2,504.0	$ 2,454.4
Common Equity Tier One Risk Based Capital To Risk Weighted Assets		12.16%	11.08%
Adequately capitalized Basel III phase-in schedule, amount		$ 926.9	$ 996.5
Adequately capitalized Basel III phase-in schedule, ratio (percent)		4.50%	4.50%
Adequately capitalized Basel III fully phased-in, amount		$ 1,441.8	$ 1,550.1
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		$ 1,338.9	$ 1,439.4
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets		6.50%	6.50%
Leverage capital ratio:
Actual amount		$ 2,504.0	$ 2,454.4
Tier One Leverage Capital to Average Assets		0.0871	0.0822
Adequately capitalized, amount		$ 1,149.4	$ 1,193.9
Adequately capitalized, ratio (percent)		0.0400	0.0400
Well capitalized, amount		$ 1,436.8	$ 1,492.3
Well capitalized, ratio (percent)		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		$ 2,691.5	$ 2,662.0
Capital to Risk Weighted Assets		0.1310	0.1204
Adequately capitalized Basel III phase-in schedule, amount		$ 1,644.1	$ 1,768.3
Adequately capitalized Basel III phase-in schedule, ratio (percent)		0.0800	0.0800
Adequately capitalized Basel III fully phased-in, amount		$ 2,157.9	$ 2,320.8
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets		10.50%	10.50%
Well capitalized, amount		$ 2,055.2	$ 2,210.3
Capital Required to be Well Capitalized to Risk Weighted Assets		0.1000	0.1000
Tier 1 risk-based capital:
Actual amount		$ 2,490.3	$ 2,433.0
Tier One Risk Based Capital to Risk Weighted Assets		0.1212	0.1101
Adequately capitalized Basel III phase-in schedule, amount		$ 1,233.1	$ 1,326.2
Adequately capitalized Basel III phase-in schedule, ratio (percent)		0.0600	0.0600
Adequately capitalized Basel III fully phased-in, amount		$ 1,746.9	$ 1,878.8
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets		8.50%	8.50%
Well capitalized, amount		$ 1,644.1	$ 1,768.3
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		$ 2,490.3	$ 2,433.0
Common Equity Tier One Risk Based Capital To Risk Weighted Assets		12.12%	11.01%
Adequately capitalized Basel III phase-in schedule, amount		$ 924.8	$ 994.6
Adequately capitalized Basel III phase-in schedule, ratio (percent)		4.50%	4.50%
Adequately capitalized Basel III fully phased-in, amount		$ 1,438.6	$ 1,547.2
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		$ 1,335.8	$ 1,436.7
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets		6.50%	6.50%
Leverage capital ratio:
Actual amount		$ 2,490.3	$ 2,433.0
Tier One Leverage Capital to Average Assets		0.0868	0.0816
Adequately capitalized, amount		$ 1,147.5	$ 1,192.2
Adequately capitalized, ratio (percent)		0.0400	0.0400
Well capitalized, amount		$ 1,434.4	$ 1,490.2
Well capitalized, ratio (percent)		0.0500	0.0500