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Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions |  | 12 Months Ended |
--- | --- | --- | ---
Jun. 07, 2021 | 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 |  | $ 647.4 |
Available-for-Sale: |  |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 4,859.3 | $ 3,932.7
Gross Unrealized Gains |  | 20.6 | 81.7
Gross Unrealized Losses |  | (59.4) | (5.7)
Estimated Fair Value |  | 4,820.5 | 4,008.7
Held-to-Maturity: |  |  |
Amortized Cost |  | 1,687.6 | 51.6
Gross Unrealized Gains |  | 15.6 | 3.4
Gross Unrealized Losses |  | 35.7 | 0.0
Estimated Fair Value |  | 1,667.5 | 55.0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value |  | 742.8 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost |  | 696.4 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value |  | 687.2 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost |  | 1,482.4 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value |  | 1,577.4 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost |  | 374.5 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value |  | 363.6 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost |  | 1,313.4 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value |  | 1,165.6 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost |  | 275.6 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value |  | 276.2 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost |  | 1,416.1 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value |  | 1,334.7 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost |  | 341.1 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value |  | 340.5 |
Debt Securities, Available-for-Sale and Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss | $ 646.7 |  |
Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Allowance for Credit Loss, Expense | 672.2 |  |
US Treasury notes |  |  |
Available-for-Sale: |  |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 697.6 |
Gross Unrealized Gains |  | 0.0 |
Gross Unrealized Losses |  | (12.9) |
Estimated Fair Value |  | 684.7 |
State, county and municipal securities |  |  |
Available-for-Sale: |  |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 434.7 | 462.1
Gross Unrealized Gains |  | 2.1 | 4.8
Gross Unrealized Losses |  | (9.3) | (1.0)
Estimated Fair Value |  | 427.5 | 465.9
Held-to-Maturity: |  |  |
Amortized Cost |  | 67.6 | 46.6
Gross Unrealized Gains |  | 2.0 | 3.2
Gross Unrealized Losses |  | 0.4 | 0.0
Estimated Fair Value |  | 69.2 | 49.8
US Government Agencies Debt Securities [Member] |  |  |
Available-for-Sale: |  |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 356.0 | 332.9
Gross Unrealized Gains |  | 0.1 | 1.0
Gross Unrealized Losses |  | (9.2) | (2.0)
Estimated Fair Value |  | 346.9 | 331.9
Corporate Debt Securities [Member] |  |  |
Available-for-Sale: |  |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 271.1 | 295.8
Gross Unrealized Gains |  | 3.0 | 6.5
Gross Unrealized Losses |  | (3.6) | (0.1)
Estimated Fair Value |  | 270.5 | 302.2
Held-to-Maturity: |  |  |
Amortized Cost |  | 11.0 | 3.9
Gross Unrealized Gains |  | 0.4 | 0.1
Gross Unrealized Losses |  | 0.0 | 0.0
Estimated Fair Value |  | 11.4 | 4.0
Debt and Equity Securities, Unrealized Gain (Loss) |  | 20.1 |
Debt Securities, Available-for-Sale and Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss | 17.3 |  |
Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Allowance for Credit Loss, Expense | 17.7 |  |
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] |  |  |
Available-for-Sale: |  |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 2,027.3 | 2,830.8
Gross Unrealized Gains |  | 14.1 | 69.3
Gross Unrealized Losses |  | (23.3) | (2.5)
Estimated Fair Value |  | 2,018.1 | 2,897.6
Held-to-Maturity: |  |  |
Amortized Cost |  | 1,609.0 | 1.0
Gross Unrealized Gains |  | 13.2 | 0.1
Gross Unrealized Losses |  | 35.3 | 0.0
Estimated Fair Value |  | 1,586.9 | 1.1
Debt Securities, Available-for-Sale and Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss | 629.4 |  |
Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Allowance for Credit Loss, Expense | $ 654.5 |  |
Mortgage-backed Securities, Issued by Private Enterprises [Member] |  |  |
Available-for-Sale: |  |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 174.4 | 10.9
Gross Unrealized Gains |  | 0.1 | 0.1
Gross Unrealized Losses |  | (1.1) | (0.1)
Estimated Fair Value |  | 173.4 | 10.9
Other Debt Obligations [Member] |  |  |
Available-for-Sale: |  |  |
Debt Securities, Available-for-sale, Amortized Cost |  |  | 0.2
Gross Unrealized Gains |  |  | 0.0
Gross Unrealized Losses |  |  | 0.0
Estimated Fair Value |  |  | 0.2
Other Investments |  |  |
Held-to-Maturity: |  |  |
Amortized Cost |  |  | 0.1
Gross Unrealized Gains |  |  | 0.0
Gross Unrealized Losses |  |  | 0.0
Estimated Fair Value |  |  | $ 0.1
Collateralized Loan Obligations |  |  |
Available-for-Sale: |  |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 898.2 |
Gross Unrealized Gains |  | 1.2 |
Gross Unrealized Losses |  | 0.0 |
Estimated Fair Value |  | 899.4 |
US States And Political Subdivisions Member, General Obligation Securities |  |  |
Available-for-Sale: |  |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 55.9 |
Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming | State, county and municipal securities |  |  |
Available-for-Sale: |  |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 40.7 |
Callable Within One Year [Member] |  |  |
Schedule of Available for Sale and Held-to-Maturity Securities |  |  |
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis |  | 236.1 |
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Fair Value |  | 235.7 |
Held-to-Maturity: |  |  |
Investment Securities, Callable Structured, Amortized Cost Basis |  | $ 0.0 |

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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in next rolling fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date. + References No definition available. + Details
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- Definition Amortized cost, before allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity) and investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27232-111563 + Details
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- Definition Amount of expense for allowance for credit loss on investment in debt security measured at fair value, with change in fair value recognized in other comprehensive income (available-for-sale) from transfer to investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 8B -URI http://asc.fasb.org/extlink&oid=124260329&loc=SL120268709-111562 + Details
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