Regulatory Capital (Details) $ in Millions	Jan. 01, 2020 USD ($)	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained Earnings (Accumulated Deficit)		$ 1,005.2	$ 962.1
Total risk-based capital:
Actual, Amount		$ 1,615.5	$ 1,575.7
Actual, Ratio		0.1389	0.1419
Minimum Required for Capital Adequacy Purposes, Amount		$ 930.3	$ 888.3
Minimum Required for Capital Adequacy Purposes, Ratio		0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,221.0	$ 1,165.8
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		10.50%	10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,162.8	$ 1,110.3
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.1000	0.1000
Tier 1 risk-based capital:
Actual, Amount		$ 1,415.6	$ 1,369.0
Actual, Ratio		0.1217	0.1233
Minimum Required for Capital Adequacy Purposes, Amount		$ 697.7	$ 666.2
Minimum Required for Capital Adequacy Purposes, Ratio		0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 988.4	$ 943.8
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		8.50%	8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 930.3	$ 888.3
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0800	0.0800
Common equity tier 1 risk-based capital:
Actual, Amount		$ 1,331.5	$ 1,284.9
Actual, Ratio		11.45%	11.57%
Minimum Required for Capital Adequacy Purposes, Amount		$ 523.3	$ 499.6
Minimum Required for Capital Adequacy Purposes, Ratio		4.50%	4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 814.0	$ 777.2
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		7.00%	7.00%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 755.8	$ 721.7
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		6.50%	6.50%
Leverage capital ratio:
Actual, Amount		$ 1,415.6	$ 1,369.0
Actual, Ratio		0.0784	0.0816
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum		$ 722.0	$ 671.0
Capital Adequacy Purposes and Conservation Buffer, Ratio		0.0400	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 902.5	$ 838.7
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0500	0.0500
Income Tax Effects Allocated Directly to Equity, Cumulative Effect of Change in Accounting Principle	$ 24.1
FIB
Total risk-based capital:
Actual, Amount		$ 1,445.4	$ 1,426.8
Actual, Ratio		0.1247	0.1289
Minimum Required for Capital Adequacy Purposes, Amount		$ 927.5	$ 885.6
Minimum Required for Capital Adequacy Purposes, Ratio		0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,217.4	$ 1,162.3
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		10.50%	10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,159.4	$ 1,107.0
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.1000	0.1000
Tier 1 risk-based capital:
Actual, Amount		$ 1,345.5	$ 1,320.1
Actual, Ratio		0.1160	0.1193
Minimum Required for Capital Adequacy Purposes, Amount		$ 695.7	$ 664.2
Minimum Required for Capital Adequacy Purposes, Ratio		0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 985.5	$ 940.9
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		8.50%	8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 927.5	$ 885.6
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0800	0.0800
Common equity tier 1 risk-based capital:
Actual, Amount		$ 1,345.5	$ 1,320.1
Actual, Ratio		11.60%	11.93%
Minimum Required for Capital Adequacy Purposes, Amount		$ 521.7	$ 498.1
Minimum Required for Capital Adequacy Purposes, Ratio		4.50%	4.50%
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 811.6	$ 774.9
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		7.00%	7.00%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 753.6	$ 719.5
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		6.50%	6.50%
Leverage capital ratio:
Actual, Amount		$ 1,345.5	$ 1,320.1
Actual, Ratio		0.0747	0.0788
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum		$ 720.2	$ 669.7
Capital Adequacy Purposes and Conservation Buffer, Ratio		0.0400	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 900.3	$ 837.2
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0500	0.0500