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Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
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 | $ 687.6 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 5.7 | $ 7.2
Available-for-sale Securities, Gross Realized Gains | 0.0 |
Available-for-sale Securities, Gross Realized Losses |  | 0.0
Available-for-Sale: |  |
Amortized Cost | 3,932.7 | 2,945.5
Gross Unrealized Gains | 81.7 | 21.7
Gross Unrealized Losses | (5.7) | (7.2)
Estimated Fair Value | 4,008.7 | 2,960.0
Held-to-Maturity: |  |
Amortized Cost | 51.6 | 92.3
Gross Unrealized Gains | 3.4 | 2.2
Gross Unrealized Losses | 0.0 | 0.0
Estimated Fair Value | 55.0 | 94.5
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value | 991.4 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost | 8.3 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value | 8.4 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost | 1,587.7 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value | 1,362.6 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost | 27.8 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value | 29.0 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost | 736.2 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value | 996.3 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost | 14.1 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value | 16.2 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost | 921.2 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value | 658.4 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost | 1.4 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value | 1.4 |
US Treasury notes |  |
Available-for-Sale: |  |
Amortized Cost |  | 9.0
Gross Unrealized Gains |  | 0.0
Gross Unrealized Losses |  | 0.0
Estimated Fair Value |  | 9.0
State, county and municipal securities |  |
Schedule of Available for Sale and Held-to-Maturity Securities |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 1.0 |
Available-for-Sale: |  |
Amortized Cost | 462.1 |
Gross Unrealized Gains | 4.8 | 0.8
Gross Unrealized Losses | (1.0) | 0.0
Estimated Fair Value | 465.9 | 80.9
Held-to-Maturity: |  |
Amortized Cost | 46.6 | 57.3
Gross Unrealized Gains | 3.2 | 2.1
Gross Unrealized Losses | 0.0 | 0.0
Estimated Fair Value | 49.8 | 59.4
Available-for-sale securities, amortized cost basis | 38.6 | 80.1
US Government Agencies Debt Securities [Member] |  |
Schedule of Available for Sale and Held-to-Maturity Securities |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 2.0 | 0.8
Available-for-Sale: |  |
Amortized Cost | 332.9 | 367.5
Gross Unrealized Gains | 1.0 | 0.1
Gross Unrealized Losses | (2.0) | (0.8)
Estimated Fair Value | 331.9 | 366.8
Held-to-Maturity: |  |
Amortized Cost |  | 19.8
Gross Unrealized Gains |  | 0.0
Gross Unrealized Losses |  | 0.0
Estimated Fair Value |  | 19.8
Corporate Debt Securities [Member] |  |
Schedule of Available for Sale and Held-to-Maturity Securities |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 0.1 |
Available-for-Sale: |  |
Amortized Cost | 295.8 | 134.5
Gross Unrealized Gains | 6.5 | 1.2
Gross Unrealized Losses | (0.1) | 0.0
Estimated Fair Value | 302.2 | 135.7
Held-to-Maturity: |  |
Amortized Cost | 3.9 | 13.9
Gross Unrealized Gains | 0.1 | 0.1
Gross Unrealized Losses | 0.0 | 0.0
Estimated Fair Value | 4.0 | 14.0
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] |  |
Schedule of Available for Sale and Held-to-Maturity Securities |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 2.5 | 6.0
Available-for-Sale: |  |
Amortized Cost | 2,830.8 | 2,303.6
Gross Unrealized Gains | 69.3 | 19.6
Gross Unrealized Losses | (2.5) | (6.0)
Estimated Fair Value | 2,897.6 | 2,317.2
Held-to-Maturity: |  |
Amortized Cost | 1.0 | 1.2
Gross Unrealized Gains | 0.1 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Estimated Fair Value | 1.1 | 1.2
Mortgage-backed Securities, Issued by Private Enterprises [Member] |  |
Schedule of Available for Sale and Held-to-Maturity Securities |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 0.1 | 0.4
Available-for-Sale: |  |
Amortized Cost | 10.9 | 47.6
Gross Unrealized Gains | 0.1 | 0.0
Gross Unrealized Losses | (0.1) | (0.4)
Estimated Fair Value | 10.9 | 47.2
Other Debt Obligations [Member] |  |
Available-for-Sale: |  |
Amortized Cost | 0.2 | 3.2
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Estimated Fair Value | 0.2 | 3.2
Other Investments |  |
Held-to-Maturity: |  |
Amortized Cost | 0.1 | 0.1
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Estimated Fair Value | 0.1 | $ 0.1
Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming | State, county and municipal securities |  |
Held-to-Maturity: |  |
Available-for-sale securities, amortized cost basis | 26.0 |
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 | 282.4 |
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Fair Value | 281.7 |
Held-to-Maturity: |  |
Investment Securities, Callable Structured, Amortized Cost Basis | $ 0.0 |

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