Regulatory Capital - Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) $ in Millions	Jan. 01, 2020 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Total risk-based capital:
Actual amount		$ 2,941.1	$ 2,875.8
Capital to Risk Weighted Assets		0.1328	0.1248
Adequately capitalized Basel III phase-in schedule, amount		$ 1,771.6	$ 1,843.2
Adequately capitalized Basel III phase-in schedule, ratio (percent)		0.0800	0.0800
Adequately capitalized Basel III fully phased-in, amount		$ 2,325.2	$ 2,419.2
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets		10.50%	10.50%
Well capitalized, amount		$ 2,214.5	$ 2,304.0
Capital Required to be Well Capitalized to Risk Weighted Assets		0.1000	0.1000
Tier 1 risk-based capital:
Actual amount		$ 2,454.4	$ 2,408.8
Tier One Risk Based Capital to Risk Weighted Assets		0.1108	0.1045
Adequately capitalized Basel III phase-in schedule, amount		$ 1,328.7	$ 1,382.4
Adequately capitalized Basel III phase-in schedule, ratio (percent)		0.0600	0.0600
Adequately capitalized Basel III fully phased-in, amount		$ 1,882.3	$ 1,958.4
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets		8.50%	8.50%
Well capitalized, amount		$ 1,771.6	$ 1,843.2
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		$ 2,454.4	$ 2,408.8
Common Equity Tier One Risk Based Capital To Risk Weighted Assets		11.08%	10.45%
Adequately capitalized Basel III phase-in schedule, amount		$ 996.5	$ 1,036.8
Adequately capitalized Basel III phase-in schedule, ratio (percent)		4.50%	4.50%
Adequately capitalized Basel III fully phased-in, amount		$ 1,550.1	$ 1,612.8
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		$ 1,439.4	$ 1,497.6
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets		6.50%	6.50%
Leverage capital ratio:
Actual amount		$ 2,454.4	$ 2,408.8
Tier One Leverage Capital to Average Assets		0.0822	0.0775
Adequately capitalized, amount		$ 1,193.9	$ 1,242.9
Adequately capitalized, ratio (percent)		0.0400	0.0400
Well capitalized, amount		$ 1,492.3	$ 1,553.6
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		$ 2,662.0	$ 2,713.5
Capital to Risk Weighted Assets		0.1204	0.1180
Adequately capitalized Basel III phase-in schedule, amount		$ 1,768.3	$ 1,839.6
Adequately capitalized Basel III phase-in schedule, ratio (percent)		0.0800	0.0800
Adequately capitalized Basel III fully phased-in, amount		$ 2,320.8	$ 2,414.5
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets		10.50%	10.50%
Well capitalized, amount		$ 2,210.3	$ 2,299.5
Capital Required to be Well Capitalized to Risk Weighted Assets		0.1000	0.1000
Tier 1 risk-based capital:
Actual amount		$ 2,433.0	$ 2,504.1
Tier One Risk Based Capital to Risk Weighted Assets		0.1101	0.1089
Adequately capitalized Basel III phase-in schedule, amount		$ 1,326.2	$ 1,379.7
Adequately capitalized Basel III phase-in schedule, ratio (percent)		0.0600	0.0600
Adequately capitalized Basel III fully phased-in, amount		$ 1,878.8	$ 1,954.6
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets		8.50%	8.50%
Well capitalized, amount		$ 1,768.3	$ 1,839.6
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		$ 2,433.0	$ 2,504.1
Common Equity Tier One Risk Based Capital To Risk Weighted Assets		11.01%	10.89%
Adequately capitalized Basel III phase-in schedule, amount		$ 994.6	$ 1,034.8
Adequately capitalized Basel III phase-in schedule, ratio (percent)		4.50%	4.50%
Adequately capitalized Basel III fully phased-in, amount		$ 1,547.2	$ 1,609.7
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		$ 1,436.7	$ 1,494.7
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets		6.50%	6.50%
Leverage capital ratio:
Actual amount		$ 2,433.0	$ 2,504.1
Tier One Leverage Capital to Average Assets		0.0816	0.0807
Adequately capitalized, amount		$ 1,192.2	$ 1,241.1
Adequately capitalized, ratio (percent)		0.0400	0.0400
Well capitalized, amount		$ 1,490.2	$ 1,551.4
Well capitalized, ratio (percent)		0.0500	0.0500