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Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Schedule of Available for Sale and Held-to-Maturity Securities |  |  |
Available-for-sale Securities, Gross Realized Gains | $ 0.0 | $ 1.1 | $ 0.4
Available-for-sale Securities, Gross Realized Losses | (0.1) | (0.4) | $ (0.1)
Available-for-Sale: |  |  |
Amortized Cost | 2,305.2 | 2,229.3 |
Gross Unrealized Gains | 2.6 | 3.9 |
Gross Unrealized Losses | (37.1) | (24.5) |
Estimated Fair Value | 2,270.7 | 2,208.7 |
Held-to-Maturity: |  |  |
Amortized Cost | 406.8 | 484.5 |
Gross Unrealized Gains | 2.2 | 11.5 |
Gross Unrealized Losses | (8.3) | (12.7) |
Estimated Fair Value | 400.7 | 483.3 |
US Treasury notes |  |  |
Available-for-Sale: |  |  |
Amortized Cost | 2.6 | 3.2 |
Gross Unrealized Gains | 0.0 | 0.0 |
Gross Unrealized Losses | 0.0 | 0.0 |
Estimated Fair Value | 2.6 | 3.2 |
State, county and municipal securities |  |  |
Held-to-Maturity: |  |  |
Amortized Cost | 150.9 | 172.4 |
Gross Unrealized Gains | 1.8 | 2.6 |
Gross Unrealized Losses | (0.9) | (0.6) |
Estimated Fair Value | 151.8 | 174.4 |
Available-for-sale securities, amortized cost basis | 115.5 |  |
Obligations of U.S. government agencies |  |  |
Available-for-Sale: |  |  |
Amortized Cost | 569.3 | 569.5 |
Gross Unrealized Gains | 0.1 | 0.0 |
Gross Unrealized Losses | (10.2) | (8.0) |
Estimated Fair Value | 559.2 | 561.5 |
Held-to-Maturity: |  |  |
Amortized Cost | 19.8 | 19.8 |
Gross Unrealized Gains | 0.0 | 0.0 |
Gross Unrealized Losses | (0.3) | (0.2) |
Estimated Fair Value | 19.5 | 19.6 |
Corporate Debt Securities [Member] |  |  |
Available-for-Sale: |  |  |
Amortized Cost | 92.9 | 88.0 |
Gross Unrealized Gains | 0.0 | 0.1 |
Gross Unrealized Losses | (1.0) | (0.3) |
Estimated Fair Value | 91.9 | 87.8 |
Held-to-Maturity: |  |  |
Amortized Cost | 46.3 | 61.6 |
Gross Unrealized Gains | 0.1 | 0.1 |
Gross Unrealized Losses | (0.6) | (0.3) |
Estimated Fair Value | 45.8 | 61.4 |
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations |  |  |
Available-for-Sale: |  |  |
Amortized Cost | 1,566.4 | 1,474.1 |
Gross Unrealized Gains | 2.5 | 3.8 |
Gross Unrealized Losses | (24.1) | (15.4) |
Estimated Fair Value | 1,544.8 | 1,462.5 |
Held-to-Maturity: |  |  |
Amortized Cost | 189.7 | 230.5 |
Gross Unrealized Gains | 0.3 | 8.8 |
Gross Unrealized Losses | (6.5) | (11.6) |
Estimated Fair Value | 183.5 | 227.7 |
Private mortgage-backed securities |  |  |
Available-for-Sale: |  |  |
Amortized Cost | 72.0 | 91.5 |
Gross Unrealized Gains | 0.0 | 0.0 |
Gross Unrealized Losses | (1.8) | (0.8) |
Estimated Fair Value | 70.2 | 90.7 |
Other Investments |  |  |
Available-for-Sale: |  |  |
Amortized Cost | 2.0 | 3.0 |
Gross Unrealized Gains | 0.0 | 0.0 |
Gross Unrealized Losses | 0.0 | 0.0 |
Estimated Fair Value | 2.0 | 3.0 |
Other Investments |  |  |
Held-to-Maturity: |  |  |
Amortized Cost | 0.1 | 0.2 |
Gross Unrealized Gains | 0.0 | 0.0 |
Gross Unrealized Losses | 0.0 | 0.0 |
Estimated Fair Value | 0.1 | $ 0.2 |
Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming | State, county and municipal securities |  |  |
Held-to-Maturity: |  |  |
Available-for-sale securities, amortized cost basis | $ 66.8 |  |

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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