Regulatory Capital - Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Total risk-based capital:
Actual amount	$ 1,575.7	$ 1,495.3
Capital to Risk Weighted Assets	14.19%	14.10%
Adequately capitalized Basel III phase-in schedule, amount	$ 888.3	$ 848.5
Adequately capitalized Basel III phase-in schedule, ratio (percent)	8.00%	8.00%
Adequately capitalized Basel III fully phased-in, amount	$ 1,165.8	$ 1,113.6
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	10.50%	10.50%
Well capitalized, amount	$ 1,110.3	$ 1,060.6
Capital Required to be Well Capitalized to Risk Weighted Assets	10.00%	10.00%
Tier 1 risk-based capital:
Actual amount	$ 1,369.0	$ 1,422.3
Tier One Risk Based Capital to Risk Weighted Assets	12.33%	13.41%
Adequately capitalized Basel III phase-in schedule, amount	$ 666.2	$ 636.3
Adequately capitalized Basel III phase-in schedule, ratio (percent)	6.00%	6.00%
Adequately capitalized Basel III fully phased-in, amount	$ 943.8	$ 901.5
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	8.50%	8.50%
Well capitalized, amount	$ 888.3	$ 848.5
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	8.00%	8.00%
Common Equity Tier One Capital [Abstract]
Actual amount	$ 1,284.9	$ 1,338.2
Common Equity Tier One Risk Based Capital To Risk Weighted Assets	11.57%	12.62%
Adequately capitalized Basel III phase-in schedule, amount	$ 499.6	$ 477.3
Adequately capitalized Basel III phase-in schedule, ratio (percent)	4.50%	4.50%
Adequately capitalized Basel III fully phased-in, amount	$ 777.2	$ 742.4
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	$ 721.7	$ 689.4
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets	6.50%	6.50%
Leverage capital ratio:
Actual amount	$ 1,369.0	$ 1,422.3
Tier One Leverage Capital to Average Assets	8.16%	10.13%
Adequately capitalized, amount	$ 671.0	$ 561.6
Adequately capitalized, ratio (percent)	4.00%	4.00%
Well capitalized, amount	$ 838.7	$ 702.0
Well capitalized, ratio (percent)	5.00%	5.00%
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	$ (24.1)
Retained Earnings [Member]
Leverage capital ratio:
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(24.1)
FIB
Total risk-based capital:
Actual amount	$ 1,426.8	$ 1,321.4
Capital to Risk Weighted Assets	12.89%	12.50%
Adequately capitalized Basel III phase-in schedule, amount	$ 885.6	$ 845.8
Adequately capitalized Basel III phase-in schedule, ratio (percent)	8.00%	8.00%
Adequately capitalized Basel III fully phased-in, amount	$ 1,162.3	$ 1,110.1
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	10.50%	10.50%
Well capitalized, amount	$ 1,107.0	$ 1,057.2
Capital Required to be Well Capitalized to Risk Weighted Assets	10.00%	10.00%
Tier 1 risk-based capital:
Actual amount	$ 1,320.1	$ 1,248.4
Tier One Risk Based Capital to Risk Weighted Assets	11.93%	11.81%
Adequately capitalized Basel III phase-in schedule, amount	$ 664.2	$ 634.3
Adequately capitalized Basel III phase-in schedule, ratio (percent)	6.00%	6.00%
Adequately capitalized Basel III fully phased-in, amount	$ 940.9	$ 898.6
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	8.50%	8.50%
Well capitalized, amount	$ 885.6	$ 845.8
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	8.00%	8.00%
Common Equity Tier One Capital [Abstract]
Actual amount	$ 1,320.1	$ 1,248.4
Common Equity Tier One Risk Based Capital To Risk Weighted Assets	11.93%	11.81%
Adequately capitalized Basel III phase-in schedule, amount	$ 498.1	$ 475.7
Adequately capitalized Basel III phase-in schedule, ratio (percent)	4.50%	4.50%
Adequately capitalized Basel III fully phased-in, amount	$ 774.9	$ 740.0
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	$ 719.5	$ 687.2
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets	6.50%	6.50%
Leverage capital ratio:
Actual amount	$ 1,320.1	$ 1,248.4
Tier One Leverage Capital to Average Assets	7.88%	8.91%
Adequately capitalized, amount	$ 669.7	$ 560.4
Adequately capitalized, ratio (percent)	4.00%	4.00%
Well capitalized, amount	$ 837.2	$ 700.4
Well capitalized, ratio (percent)	5.00%	5.00%