Regulatory Capital - Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) $ in Millions	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Total risk-based capital:
Actual amount	$ 1,659.3	$ 1,575.7
Capital to Risk Weighted Assets	0.1411	0.1419
Adequately capitalized Basel III phase-in schedule, amount	$ 940.9	$ 888.3
Adequately capitalized Basel III phase-in schedule, ratio (percent)	0.0800	0.0800
Adequately capitalized Basel III fully phased-in, amount	$ 1,235.0	$ 1,165.8
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	10.50%	10.50%
Well capitalized, amount	$ 1,176.2	$ 1,110.3
Capital Required to be Well Capitalized to Risk Weighted Assets	0.1000	0.1000
Tier 1 risk-based capital:
Actual amount	$ 1,469.0	$ 1,369.0
Tier One Risk Based Capital to Risk Weighted Assets	0.1249	0.1233
Adequately capitalized Basel III phase-in schedule, amount	$ 705.7	$ 666.2
Adequately capitalized Basel III phase-in schedule, ratio (percent)	0.0600	0.0600
Adequately capitalized Basel III fully phased-in, amount	$ 999.7	$ 943.8
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	8.50%	8.50%
Well capitalized, amount	$ 940.9	$ 888.3
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	$ 1,384.8	$ 1,284.9
Common Equity Tier One Risk Based Capital To Risk Weighted Assets	11.77%	11.57%
Adequately capitalized Basel III phase-in schedule, amount	$ 529.3	$ 499.6
Adequately capitalized Basel III phase-in schedule, ratio (percent)	4.50%	4.50%
Adequately capitalized Basel III fully phased-in, amount	$ 823.3	$ 777.2
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	$ 764.5	$ 721.7
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets	6.50%	6.50%
Leverage capital ratio:
Actual amount	$ 1,469.0	$ 1,369.0
Tier One Leverage Capital to Average Assets	0.0768	0.0816
Adequately capitalized, amount	$ 765.5	$ 671.0
Adequately capitalized, ratio (percent)	0.0400	0.0400
Well capitalized, amount	$ 956.9	$ 838.7
Well capitalized, ratio (percent)	0.0500	0.0500
FIB
Total risk-based capital:
Actual amount	$ 1,472.5	$ 1,426.8
Capital to Risk Weighted Assets	0.1256	0.1289
Adequately capitalized Basel III phase-in schedule, amount	$ 938.0	$ 885.6
Adequately capitalized Basel III phase-in schedule, ratio (percent)	0.0800	0.0800
Adequately capitalized Basel III fully phased-in, amount	$ 1,231.1	$ 1,162.3
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	10.50%	10.50%
Well capitalized, amount	$ 1,172.5	$ 1,107.0
Capital Required to be Well Capitalized to Risk Weighted Assets	0.1000	0.1000
Tier 1 risk-based capital:
Actual amount	$ 1,382.2	$ 1,320.1
Tier One Risk Based Capital to Risk Weighted Assets	0.1179	0.1193
Adequately capitalized Basel III phase-in schedule, amount	$ 703.5	$ 664.2
Adequately capitalized Basel III phase-in schedule, ratio (percent)	0.0600	0.0600
Adequately capitalized Basel III fully phased-in, amount	$ 996.6	$ 940.9
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	8.50%	8.50%
Well capitalized, amount	$ 938.0	$ 885.6
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	$ 1,382.2	$ 1,320.1
Common Equity Tier One Risk Based Capital To Risk Weighted Assets	11.79%	11.93%
Adequately capitalized Basel III phase-in schedule, amount	$ 527.6	$ 498.1
Adequately capitalized Basel III phase-in schedule, ratio (percent)	4.50%	4.50%
Adequately capitalized Basel III fully phased-in, amount	$ 820.8	$ 774.9
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	$ 762.1	$ 719.5
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets	6.50%	6.50%
Leverage capital ratio:
Actual amount	$ 1,382.2	$ 1,320.1
Tier One Leverage Capital to Average Assets	0.0724	0.0788
Adequately capitalized, amount	$ 764.1	$ 669.7
Adequately capitalized, ratio (percent)	0.0400	0.0400
Well capitalized, amount	$ 955.1	$ 837.2
Well capitalized, ratio (percent)	0.0500	0.0500