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Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Jan. 01, 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 | $ 677.3 |  |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 7.2 | $ 37.1 |  |
Available-for-sale Securities, Gross Realized Gains | 0.0 |  | $ 1.1 |
Available-for-sale Securities, Gross Realized Losses |  | 0.0 | $ (0.4) |
Available-for-Sale: |  |  |  |
Amortized Cost | 2,945.5 | 2,305.2 |  |
Gross Unrealized Gains | 21.7 | 2.6 |  |
Gross Unrealized Losses | (7.2) | (37.1) |  |
Estimated Fair Value | 2,960.0 | 2,270.7 |  |
Held-to-Maturity: |  |  |  |
Amortized Cost | 92.3 | 406.8 |  | $ 281.1
Gross Unrealized Gains | 2.2 | 2.2 |  |
Gross Unrealized Losses | 0.0 | (8.3) |  |
Estimated Fair Value | 94.5 | 400.7 |  | $ 275.1
Debt Securities, Unrealized Gain (Loss) |  | 6.0 |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value | 661.4 |  |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost | 38.6 |  |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value | 38.6 |  |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost | 1,486.2 |  |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value | 1,470.9 |  |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost | 29.7 |  |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value | 30.1 |  |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost | 328.0 |  |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value | 428.2 |  |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost | 21.3 |  |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value | 23.1 |  |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost | 454.0 |  |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value | 399.5 |  |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost | 2.7 |  |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value | 2.7 |  |  |
US Treasury notes |  |  |  |
Available-for-Sale: |  |  |  |
Amortized Cost | 9.0 | 2.6 |  |
Gross Unrealized Gains | 0.0 | 0.0 |  |
Gross Unrealized Losses | 0.0 | 0.0 |  |
Estimated Fair Value | 9.0 | 2.6 |  |
State, county and municipal securities |  |  |  |
Available-for-Sale: |  |  |  |
Amortized Cost | 80.1 |  |  |
Gross Unrealized Gains | 0.8 |  |  |
Gross Unrealized Losses | 0.0 |  |  |
Estimated Fair Value | 80.9 |  |  |
Held-to-Maturity: |  |  |  |
Amortized Cost | 57.3 | 150.9 |  |
Gross Unrealized Gains | 2.1 | 1.8 |  |
Gross Unrealized Losses | 0.0 | (0.9) |  |
Estimated Fair Value | 59.4 | 151.8 |  |
Available-for-sale securities, amortized cost basis | 45.9 |  |  |
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 | 0.8 | 10.2 |  |
Available-for-Sale: |  |  |  |
Amortized Cost | 367.5 | 569.3 |  |
Gross Unrealized Gains | 0.1 | 0.1 |  |
Gross Unrealized Losses | (0.8) | (10.2) |  |
Estimated Fair Value | 366.8 | 559.2 |  |
Held-to-Maturity: |  |  |  |
Amortized Cost | 19.8 | 19.8 |  |
Gross Unrealized Gains | 0.0 | 0.0 |  |
Gross Unrealized Losses | 0.0 | (0.3) |  |
Estimated Fair Value | 19.8 | 19.5 |  |
Corporate Debt Securities [Member] |  |  |  |
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 | 134.5 | 92.9 |  |
Gross Unrealized Gains | 1.2 | 0.0 |  |
Gross Unrealized Losses | 0.0 | (1.0) |  |
Estimated Fair Value | 135.7 | 91.9 |  |
Held-to-Maturity: |  |  |  |
Amortized Cost | 13.9 | 46.3 |  |
Gross Unrealized Gains | 0.1 | 0.1 |  |
Gross Unrealized Losses | 0.0 | (0.6) |  |
Estimated Fair Value | 14.0 | 45.8 |  |
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 | 6.0 | 24.1 |  |
Available-for-Sale: |  |  |  |
Amortized Cost | 2,303.6 | 1,566.4 |  |
Gross Unrealized Gains | 19.6 | 2.5 |  |
Gross Unrealized Losses | (6.0) | (24.1) |  |
Estimated Fair Value | 2,317.2 | 1,544.8 |  |
Held-to-Maturity: |  |  |  |
Amortized Cost | 1.2 | 189.7 |  |
Gross Unrealized Gains | 0.0 | 0.3 |  |
Gross Unrealized Losses | 0.0 | (6.5) |  |
Estimated Fair Value | 1.2 | 183.5 |  |
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 | 0.4 | 1.8 |  |
Available-for-Sale: |  |  |  |
Amortized Cost | 47.6 | 72.0 |  |
Gross Unrealized Gains | 0.0 | 0.0 |  |
Gross Unrealized Losses | (0.4) | (1.8) |  |
Estimated Fair Value | 47.2 | 70.2 |  |
Other Debt Obligations [Member] |  |  |  |
Available-for-Sale: |  |  |  |
Amortized Cost | 3.2 | 2.0 |  |
Gross Unrealized Gains | 0.0 | 0.0 |  |
Gross Unrealized Losses | 0.0 | 0.0 |  |
Estimated Fair Value | 3.2 | 2.0 |  |
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 | 28.0 |  |  |
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 | 181.2 |  |  |
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Fair Value | 181.1 |  |  |
Held-to-Maturity: |  |  |  |
Investment Securities, Callable Structured, Amortized Cost Basis | $ 0.0 |  |  |

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