Regulatory Capital - Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) $ in Millions	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Total risk-based capital:
Actual amount	$ 2,875.8	$ 1,659.3
Capital to Risk Weighted Assets	0.1248	0.1411
Adequately capitalized Basel III phase-in schedule, amount	$ 1,843.2	$ 940.9
Adequately capitalized Basel III phase-in schedule, ratio (percent)	0.0800	0.0800
Adequately capitalized Basel III fully phased-in, amount	$ 2,419.2	$ 1,235.0
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	10.50%	10.50%
Well capitalized, amount	$ 2,304.0	$ 1,176.2
Capital Required to be Well Capitalized to Risk Weighted Assets	0.1000	0.1000
Tier 1 risk-based capital:
Actual amount	$ 2,408.8	$ 1,469.0
Tier One Risk Based Capital to Risk Weighted Assets	0.1045	0.1249
Adequately capitalized Basel III phase-in schedule, amount	$ 1,382.4	$ 705.7
Adequately capitalized Basel III phase-in schedule, ratio (percent)	0.0600	0.0600
Adequately capitalized Basel III fully phased-in, amount	$ 1,958.4	$ 999.7
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	8.50%	8.50%
Well capitalized, amount	$ 1,843.2	$ 940.9
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	$ 2,408.8	$ 1,384.8
Common Equity Tier One Risk Based Capital To Risk Weighted Assets	10.45%	11.77%
Adequately capitalized Basel III phase-in schedule, amount	$ 1,036.8	$ 529.3
Adequately capitalized Basel III phase-in schedule, ratio (percent)	4.50%	4.50%
Adequately capitalized Basel III fully phased-in, amount	$ 1,612.8	$ 823.3
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	$ 1,497.6	$ 764.5
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets	6.50%	6.50%
Leverage capital ratio:
Actual amount	$ 2,408.8	$ 1,469.0
Tier One Leverage Capital to Average Assets	0.0775	0.0768
Adequately capitalized, amount	$ 1,242.9	$ 765.5
Adequately capitalized, ratio (percent)	0.0400	0.0400
Well capitalized, amount	$ 1,553.6	$ 956.9
Well capitalized, ratio (percent)	0.0500	0.0500
FIB
Total risk-based capital:
Actual amount	$ 2,713.5	$ 1,472.5
Capital to Risk Weighted Assets	0.1180	0.1256
Adequately capitalized Basel III phase-in schedule, amount	$ 1,839.6	$ 938.0
Adequately capitalized Basel III phase-in schedule, ratio (percent)	0.0800	0.0800
Adequately capitalized Basel III fully phased-in, amount	$ 2,414.5	$ 1,231.1
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	10.50%	10.50%
Well capitalized, amount	$ 2,299.5	$ 1,172.5
Capital Required to be Well Capitalized to Risk Weighted Assets	0.1000	0.1000
Tier 1 risk-based capital:
Actual amount	$ 2,504.1	$ 1,382.2
Tier One Risk Based Capital to Risk Weighted Assets	0.1089	0.1179
Adequately capitalized Basel III phase-in schedule, amount	$ 1,379.7	$ 703.5
Adequately capitalized Basel III phase-in schedule, ratio (percent)	0.0600	0.0600
Adequately capitalized Basel III fully phased-in, amount	$ 1,954.6	$ 996.6
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	8.50%	8.50%
Well capitalized, amount	$ 1,839.6	$ 938.0
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	$ 2,504.1	$ 1,382.2
Common Equity Tier One Risk Based Capital To Risk Weighted Assets	10.89%	11.79%
Adequately capitalized Basel III phase-in schedule, amount	$ 1,034.8	$ 527.6
Adequately capitalized Basel III phase-in schedule, ratio (percent)	4.50%	4.50%
Adequately capitalized Basel III fully phased-in, amount	$ 1,609.7	$ 820.8
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	$ 1,494.7	$ 762.1
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets	6.50%	6.50%
Leverage capital ratio:
Actual amount	$ 2,504.1	$ 1,382.2
Tier One Leverage Capital to Average Assets	0.0807	0.0724
Adequately capitalized, amount	$ 1,241.1	$ 764.1
Adequately capitalized, ratio (percent)	0.0400	0.0400
Well capitalized, amount	$ 1,551.4	$ 955.1
Well capitalized, ratio (percent)	0.0500	0.0500