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Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Schedule of Available for Sale and Held-to-Maturity Securities |  |  |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Amortized Cost | $ 40.1 |  |  |
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis | 1,603.3 |  |  |
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Fair Value | 1,552.2 |  |  |
Available-for-Sale: |  |  |  |
Gross Unrealized Gains | 3.2 | $ 2.3 |  |
Gross Unrealized Losses | (469.5) | (629.7) |  |
Held-to-Maturity: |  |  |  |
Debt Securities, Held-to-maturity | 3,208.7 | 3,453.7 |  |
Allowance for Credit Losses | 0.4 | 0.4 |  |
Gross Unrealized Losses | (334.3) | (400.0) |  |
Held-to-maturity investment securities, estimated fair value | 2,874.0 | 3,052.2 |  |
Debt Securities, Held-to-Maturity, Transfer, Unamortized Gain | 10.7 |  |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value | 39.4 |  |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost | 309.1 |  |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value | 307.5 |  |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost | 1,004.4 |  |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value | 938.7 |  |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost | 346.6 |  |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value | 329.4 |  |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost | 1,373.9 |  |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value | 1,255.4 |  |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost | 654.0 |  |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value | 579.1 |  |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost | 3,889.4 |  |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value | 3,608.0 |  |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost | 1,899.0 |  |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value | 1,658.0 |  |  |
Debt Securities, Held-to-maturity, Allowance for Credit Loss | (0.8) | (1.9) | $ 0.0 | $ 0.0
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss | 3,207.9 | 3,451.8 |  |
Debt and Equity Securities, Unrealized Gain (Loss) | (18.1) | (20.2) |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss | 6,307.8 | 7,573.5 |  |
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest | 0.0 | 0.0 |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss | 6,307.8 | 7,573.5 |  |
Debt Securities, Available-for-Sale, Excluding Accrued Interest | 5,841.5 | 6,946.1 |  |
US Treasury notes |  |  |  |
Available-for-Sale: |  |  |  |
Gross Unrealized Gains | 0.0 | 2.1 |  |
Gross Unrealized Losses | (25.5) | (34.5) |  |
Held-to-Maturity: |  |  |  |
Debt Securities, Held-to-maturity | 399.0 | 396.6 |  |
Allowance for Credit Losses | 0.0 | 0.0 |  |
Gross Unrealized Losses | (2.8) | (10.2) |  |
Held-to-maturity investment securities, estimated fair value | 396.2 | 386.4 |  |
Debt Securities, Held-to-maturity, Allowance for Credit Loss | 0.0 | 0.0 |  |
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss | 399.0 | 396.6 |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss | 250.2 | 675.1 |  |
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest | 0.0 | 0.0 |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss | 250.2 | 675.1 |  |
Debt Securities, Available-for-Sale, Excluding Accrued Interest | 224.7 | 642.7 |  |
State, county and municipal securities |  |  |  |
Available-for-Sale: |  |  |  |
Gross Unrealized Gains | 0.0 | 0.0 |  |
Gross Unrealized Losses | (36.9) | (50.6) |  |
Held-to-Maturity: |  |  |  |
Debt Securities, Held-to-maturity | 179.2 | 181.2 |  |
Allowance for Credit Losses | 0.2 | 0.2 |  |
Gross Unrealized Losses | (24.2) | (31.6) |  |
Held-to-maturity investment securities, estimated fair value | 155.2 | 149.7 |  |
Debt Securities, Held-to-maturity, Allowance for Credit Loss | 0.0 | (0.1) |  |
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss | 179.2 | 181.1 |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss | 256.7 | 314.3 |  |
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest | 0.0 | 0.0 |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss | 256.7 | 314.3 |  |
Debt Securities, Available-for-Sale, Excluding Accrued Interest | 219.8 | 263.7 |  |
US Government Agencies Debt Securities [Member] |  |  |  |
Available-for-Sale: |  |  |  |
Gross Unrealized Gains | 0.0 | 0.0 |  |
Gross Unrealized Losses | (10.9) | (17.3) |  |
Held-to-Maturity: |  |  |  |
Debt Securities, Held-to-maturity | 354.5 | 351.7 |  |
Allowance for Credit Losses | 0.0 | 0.0 |  |
Gross Unrealized Losses | (42.4) | (49.6) |  |
Held-to-maturity investment securities, estimated fair value | 312.1 | 302.1 |  |
Debt Securities, Held-to-maturity, Allowance for Credit Loss | 0.0 | 0.0 |  |
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss | 354.5 | 351.7 |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss | 179.4 | 216.2 |  |
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest | 0.0 | 0.0 |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss | 179.4 | 216.2 |  |
Debt Securities, Available-for-Sale, Excluding Accrued Interest | 168.5 | 198.9 |  |
Corporate Debt Securities [Member] |  |  |  |
Available-for-Sale: |  |  |  |
Gross Unrealized Gains | 0.0 | 0.0 |  |
Gross Unrealized Losses | (24.4) | (30.8) |  |
Held-to-Maturity: |  |  |  |
Debt Securities, Held-to-maturity | 57.0 | 80.1 |  |
Allowance for Credit Losses | 0.0 | 0.0 |  |
Gross Unrealized Losses | (6.0) | (7.0) |  |
Held-to-maturity investment securities, estimated fair value | 50.2 | 71.3 |  |
Debt Securities, Held-to-maturity, Allowance for Credit Loss | (0.8) | (1.8) |  |
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss | 56.2 | 78.3 |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss | 260.8 | 272.3 |  |
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest | 0.0 | 0.0 |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss | 260.8 | 272.3 |  |
Debt Securities, Available-for-Sale, Excluding Accrued Interest | 236.4 | 241.5 |  |
Mortgage-backed Securities, Issued by Private Enterprises [Member] |  |  |  |
Available-for-Sale: |  |  |  |
Gross Unrealized Gains | 0.0 | 0.0 |  |
Gross Unrealized Losses | (30.9) | (36.9) |  |
Held-to-Maturity: |  |  |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss | 241.3 | 264.9 |  |
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest | 0.0 | 0.0 |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss | 241.3 | 264.9 |  |
Debt Securities, Available-for-Sale, Excluding Accrued Interest | 210.4 | 228.0 |  |
Collateralized Loan Obligations |  |  |  |
Available-for-Sale: |  |  |  |
Gross Unrealized Gains | 0.1 | 0.0 |  |
Gross Unrealized Losses | (2.3) | (33.6) |  |
Held-to-Maturity: |  |  |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss | 1,121.9 | 1,145.2 |  |
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest | 0.0 | 0.0 |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss | 1,121.9 | 1,145.2 |  |
Debt Securities, Available-for-Sale, Excluding Accrued Interest | 1,119.7 | 1,111.6 |  |
Residential Mortgage-Backed Securities |  |  |  |
Available-for-Sale: |  |  |  |
Gross Unrealized Gains | 1.2 | 0.0 |  |
Gross Unrealized Losses | (130.6) | (167.7) |  |
Held-to-Maturity: |  |  |  |
Debt Securities, Held-to-maturity | 1,232.6 | 1,391.5 |  |
Allowance for Credit Losses | 0.0 | 0.0 |  |
Gross Unrealized Losses | (137.0) | (166.6) |  |
Held-to-maturity investment securities, estimated fair value | 1,095.6 | 1,224.9 |  |
Debt Securities, Held-to-maturity, Allowance for Credit Loss | 0.0 | 0.0 |  |
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss | 1,232.6 | 1,391.5 |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss | 1,496.3 | 1,726.0 |  |
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest | 0.0 | 0.0 |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss | 1,496.3 | 1,726.0 |  |
Debt Securities, Available-for-Sale, Excluding Accrued Interest | 1,366.9 | 1,558.3 |  |
Collateralized Mortgage Obligations |  |  |  |
Available-for-Sale: |  |  |  |
Gross Unrealized Gains | 1.3 | 0.0 |  |
Gross Unrealized Losses | (120.3) | (141.3) |  |
Held-to-Maturity: |  |  |  |
Debt Securities, Held-to-maturity | 475.9 | 525.8 |  |
Allowance for Credit Losses | 0.2 | 0.2 |  |
Gross Unrealized Losses | (69.0) | (76.7) |  |
Held-to-maturity investment securities, estimated fair value | 407.1 | 449.3 |  |
Debt Securities, Held-to-maturity, Allowance for Credit Loss | 0.0 | 0.0 |  |
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss | 475.9 | 525.8 |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss | 1,308.5 | 1,491.5 |  |
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest | 0.0 | 0.0 |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss | 1,308.5 | 1,491.5 |  |
Debt Securities, Available-for-Sale, Excluding Accrued Interest | 1,189.5 | 1,350.2 |  |
Commercial Mortgage Backed Securities |  |  |  |
Available-for-Sale: |  |  |  |
Gross Unrealized Gains | 0.6 | 0.2 |  |
Gross Unrealized Losses | (87.7) | (117.0) |  |
Held-to-Maturity: |  |  |  |
Debt Securities, Held-to-maturity | 510.5 | 526.8 |  |
Allowance for Credit Losses | 0.0 | 0.0 |  |
Gross Unrealized Losses | (52.9) | (58.3) |  |
Held-to-maturity investment securities, estimated fair value | 457.6 | 468.5 |  |
Debt Securities, Held-to-maturity, Allowance for Credit Loss | 0.0 | 0.0 |  |
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss | 510.5 | 526.8 |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss | 1,192.7 | 1,468.0 |  |
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest | 0.0 | 0.0 |  |
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss | 1,192.7 | 1,468.0 |  |
Debt Securities, Available-for-Sale, Excluding Accrued Interest | $ 1,105.6 | 1,351.2 |  |
US agency, residential and commercial MBS and CMOs |  |  |  |
Held-to-Maturity: |  |  |  |
Debt Securities, Held-to-Maturity, Transfer, Unamortized Gain |  | $ 14.2 |  |

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