Regulatory Capital (Details) $ in Millions	Jan. 01, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Income Tax Effects Allocated Directly to Equity, Cumulative Effect of Change in Accounting Principle	$ (24.1)
Total risk-based capital:
Actual, Amount		$ 1,460.3	$ 1,495.3
Actual, Ratio		0.1367	0.1410
Minimum Required for Capital Adequacy Purposes, Amount		$ 854.8	$ 848.5
Minimum Required for Capital Adequacy Purposes, Ratio		0.08000	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,121.9	$ 1,113.6
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		10.50%	10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,068.5	$ 1,060.6
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.1000	0.1000
Tier 1 risk-based capital:
Actual, Amount		$ 1,367.7	$ 1,422.3
Actual, Ratio		0.1280	0.1341
Minimum Required for Capital Adequacy Purposes, Amount		$ 641.1	$ 636.3
Minimum Required for Capital Adequacy Purposes, Ratio		0.06000	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 908.2	$ 901.5
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		8.50%	8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 854.8	$ 848.5
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0800	0.0800
Common equity tier 1 risk-based capital:
Actual, Amount		$ 1,283.6	$ 1,338.2
Actual, Ratio		12.01%	12.62%
Minimum Required for Capital Adequacy Purposes, Amount		$ 480.8	$ 477.3
Minimum Required for Capital Adequacy Purposes, Ratio		4.50%	4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 747.9	$ 742.4
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		7.00%	7.00%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 694.5	$ 689.4
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		6.50%	6.50%
Leverage capital ratio:
Actual, Amount		$ 1,367.7	$ 1,422.3
Actual, Ratio		0.0990	0.1013
Capital Adequacy Purposes and Conservation Buffer, Amount		$ 552.5	$ 561.6
Capital Adequacy Purposes and Conservation Buffer, Ratio		0.04000	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 690.6	$ 702.0
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0500	0.0500
Accounts Receivable, Allowance for Credit Loss	29.0	$ 129.1	$ 73.0	$ 72.4	$ 73.0
FIB
Total risk-based capital:
Actual, Amount		$ 1,339.7	$ 1,321.4
Actual, Ratio		0.1258	0.1250
Minimum Required for Capital Adequacy Purposes, Amount		$ 852.3	$ 845.8
Minimum Required for Capital Adequacy Purposes, Ratio		0.08000	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,118.6	$ 1,110.1
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		10.50%	10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,065.3	$ 1,057.2
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.1000	0.1000
Tier 1 risk-based capital:
Actual, Amount		$ 1,247.1	$ 1,248.4
Actual, Ratio		0.1171	0.1181
Minimum Required for Capital Adequacy Purposes, Amount		$ 639.2	$ 634.3
Minimum Required for Capital Adequacy Purposes, Ratio		0.06000	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 905.5	$ 898.6
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		8.50%	8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 852.3	$ 845.8
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0800	0.0800
Common equity tier 1 risk-based capital:
Actual, Amount		$ 1,247.1	$ 1,248.4
Actual, Ratio		11.71%	11.81%
Minimum Required for Capital Adequacy Purposes, Amount		$ 479.4	$ 475.7
Minimum Required for Capital Adequacy Purposes, Ratio		4.50%	4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 745.7	$ 740.0
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		7.00%	7.00%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 692.5	$ 687.2
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		6.50%	6.50%
Leverage capital ratio:
Actual, Amount		$ 1,247.1	$ 1,248.4
Actual, Ratio		0.0905	0.0891
Capital Adequacy Purposes and Conservation Buffer, Amount		$ 551.2	$ 560.4
Capital Adequacy Purposes and Conservation Buffer, Ratio		0.0400	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 689.0	$ 700.4
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0500	0.0500
Retained Earnings [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Income Tax Effects Allocated Directly to Equity, Cumulative Effect of Change in Accounting Principle	(24.1)
Cumulative Effect, Period of Adoption, Adjustment
Leverage capital ratio:
Accounts Receivable, Allowance for Credit Loss	30.0		$ 30.0
Change in Accounting Principle, Indirect Effect [Member]
Leverage capital ratio:
Accounts Receivable, Allowance for Credit Loss	$ 6.5