Regulatory Capital (Details) $ in Millions	Jan. 01, 2020 USD ($)	Jun. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained Earnings (Accumulated Deficit)		$ 1,156.9	$ 1,135.1
Total risk-based capital:
Actual, Amount		$ 2,970.0	$ 2,941.1
Actual, Ratio		0.1380	0.1328
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,721.8	$ 1,771.6
Minimum Required for Capital Adequacy Purposes, Ratio		0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 2,259.9	$ 2,325.2
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		10.50%	10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 2,152.3	$ 2,214.5
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.1000	0.1000
Banking Regulation, Common Equity Tier One Risk-Based Capital, Capital Adequacy, Minimum		$ 968.5	$ 996.5
Tier 1 risk-based capital:
Actual, Amount		$ 2,482.1	$ 2,454.4
Actual, Ratio		0.1153	0.1108
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,291.4	$ 1,328.7
Minimum Required for Capital Adequacy Purposes, Ratio		0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,829.4	$ 1,882.3
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		8.50%	8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,721.8	$ 1,771.6
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0800	0.0800
Banking Regulation, Common Equity Tier One Risk-Based Capital Ratio, Well Capitalized, Minimum		0.0650	0.0650
Banking Regulation, Common Equity Tier One Risk-Based Capital Ratio, Capital Adequacy, Minimum		0.0450	0.0450
Banking Regulation, Common Equity Tier One Risk-Based Capital Ratio, Actual		0.1153	0.1108
Banking Regulation, Common Equity Tier One Risk-Based Capital, Well Capitalized, Minimum		$ 1,399.0	$ 1,439.4
Banking Regulation, Common Equity Tier One Risk-Based Capital, Actual		2,482.1	2,454.4
Common equity tier 1 risk-based capital:
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,506.6	$ 1,550.1
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		7.00%	7.00%
Leverage capital ratio:
Actual, Amount		$ 2,482.1	$ 2,454.4
Actual, Ratio		0.0844	0.0822
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum		$ 1,176.4	$ 1,193.9
Capital Adequacy Purposes and Conservation Buffer, Ratio		0.0400	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,470.6	$ 1,492.3
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		$ 2,689.2	$ 2,662.0
Actual, Ratio		0.1252	0.1204
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,718.2	$ 1,768.3
Minimum Required for Capital Adequacy Purposes, Ratio		0.0800	0.0800
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 2,255.1	$ 2,320.8
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		10.50%	10.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 2,147.7	$ 2,210.3
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.1000	0.1000
Banking Regulation, Common Equity Tier One Risk-Based Capital, Capital Adequacy, Minimum		$ 966.5	$ 994.6
Tier 1 risk-based capital:
Actual, Amount		$ 2,458.9	$ 2,433.0
Actual, Ratio		0.1145	0.1101
Minimum Required for Capital Adequacy Purposes, Amount		$ 1,288.6	$ 1,326.2
Minimum Required for Capital Adequacy Purposes, Ratio		0.0600	0.0600
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,825.6	$ 1,878.8
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		8.50%	8.50%
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,718.2	$ 1,768.3
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0800	0.0800
Banking Regulation, Common Equity Tier One Risk-Based Capital Ratio, Well Capitalized, Minimum		0.0650	0.0650
Banking Regulation, Common Equity Tier One Risk-Based Capital Ratio, Capital Adequacy, Minimum		0.0450	0.0450
Banking Regulation, Common Equity Tier One Risk-Based Capital Ratio, Actual		0.1145	0.1101
Banking Regulation, Common Equity Tier One Risk-Based Capital, Well Capitalized, Minimum		$ 1,396.0	$ 1,436.7
Banking Regulation, Common Equity Tier One Risk-Based Capital, Actual		2,458.9	2,433.0
Common equity tier 1 risk-based capital:
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount		$ 1,503.4	$ 1,547.2
For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio		7.00%	7.00%
Leverage capital ratio:
Actual, Amount		$ 2,458.9	$ 2,433.0
Actual, Ratio		0.0837	0.0816
Banking Regulation, Tier One Leverage Capital, Capital Adequacy, Minimum		$ 1,174.7	$ 1,192.2
Capital Adequacy Purposes and Conservation Buffer, Ratio		0.0400	0.0400
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount		$ 1,468.4	$ 1,490.2
Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio		0.0500	0.0500