Regulatory Capital - Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Total risk-based capital:
Actual amount	$ 1,495.3	$ 1,285.0
Capital to Risk Weighted Assets	14.10%	12.99%
Adequately capitalized Basel III phase-in schedule, amount	$ 848.5	$ 791.2
Adequately capitalized Basel III phase-in schedule, ratio (percent)	8.00%	8.00%
Adequately capitalized Basel III fully phased-in, amount	$ 1,113.6	$ 976.6
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	10.50%	9.88%
Well capitalized, amount	$ 1,060.6	$ 989.0
Capital Required to be Well Capitalized to Risk Weighted Assets	10.00%	10.00%
Tier 1 risk-based capital:
Actual amount	$ 1,422.3	$ 1,212.0
Tier One Risk Based Capital to Risk Weighted Assets	13.41%	12.26%
Adequately capitalized Basel III phase-in schedule, amount	$ 636.3	$ 593.4
Adequately capitalized Basel III phase-in schedule, ratio (percent)	6.00%	6.00%
Adequately capitalized Basel III fully phased-in, amount	$ 901.5	$ 778.8
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	8.50%	7.88%
Well capitalized, amount	$ 848.5	$ 791.2
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	$ 1,338.2	$ 1,127.8
Common Equity Tier One Risk Based Capital To Risk Weighted Assets	12.62%	11.40%
Adequately capitalized Basel III phase-in schedule, amount	$ 477.3	$ 445.0
Adequately capitalized Basel III phase-in schedule, ratio (percent)	4.50%	4.50%
Adequately capitalized Basel III fully phased-in, amount	$ 742.4	$ 630.5
Tier One Common Equity Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	7.00%	6.38%
Well capitalized, amount	$ 689.4	$ 642.8
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets	6.50%	6.50%
Leverage capital ratio:
Actual amount	$ 1,422.3	$ 1,212.0
Tier One Leverage Capital to Average Assets	10.13%	9.47%
Adequately capitalized, amount	$ 561.6	$ 511.9
Adequately capitalized, ratio (percent)	4.00%	4.00%
Well capitalized, amount	$ 702.0	$ 639.9
Well capitalized, ratio (percent)	5.00%	5.00%
FIB
Total risk-based capital:
Actual amount	$ 1,321.4	$ 1,184.5
Capital to Risk Weighted Assets	12.50%	12.01%
Adequately capitalized Basel III phase-in schedule, amount	$ 845.8	$ 788.8
Adequately capitalized Basel III phase-in schedule, ratio (percent)	8.00%	8.00%
Adequately capitalized Basel III fully phased-in, amount	$ 1,110.1	$ 973.7
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	10.50%	9.88%
Well capitalized, amount	$ 1,057.2	$ 986.0
Capital Required to be Well Capitalized to Risk Weighted Assets	10.00%	10.00%
Tier 1 risk-based capital:
Actual amount	$ 1,248.4	$ 1,111.6
Tier One Risk Based Capital to Risk Weighted Assets	11.81%	11.27%
Adequately capitalized Basel III phase-in schedule, amount	$ 634.3	$ 591.6
Adequately capitalized Basel III phase-in schedule, ratio (percent)	6.00%	6.00%
Adequately capitalized Basel III fully phased-in, amount	$ 898.6	$ 776.5
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	8.50%	7.88%
Well capitalized, amount	$ 845.8	$ 788.8
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	$ 1,248.4	$ 1,111.6
Common Equity Tier One Risk Based Capital To Risk Weighted Assets	11.81%	11.27%
Adequately capitalized Basel III phase-in schedule, amount	$ 475.7	$ 443.7
Adequately capitalized Basel III phase-in schedule, ratio (percent)	4.50%	4.50%
Adequately capitalized Basel III fully phased-in, amount	$ 740.0	$ 628.6
Tier One Common Equity Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	7.00%	6.38%
Well capitalized, amount	$ 687.2	$ 640.9
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets	6.50%	6.50%
Leverage capital ratio:
Actual amount	$ 1,248.4	$ 1,111.6
Tier One Leverage Capital to Average Assets	8.91%	8.97%
Adequately capitalized, amount	$ 560.4	$ 495.9
Adequately capitalized, ratio (percent)	4.00%	4.00%
Well capitalized, amount	$ 700.4	$ 619.8
Well capitalized, ratio (percent)	5.00%	5.00%