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Investment Securities - Realized Gains (Losses) on Investments (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Schedule of Available-for-sale Securities |  |
Gross realized gains | $ 0.0 |
Gross realized losses |  | $ 0.0
Less than 12 Months | 838.6 | 925.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 5.5 | 5.4
12 Months or More | 16.6 | 202.5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.2 | 1.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 855.2 | 1,127.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 5.7 | 7.2
US Government Agencies Debt Securities [Member] |  |
Schedule of Available-for-sale Securities |  |
Less than 12 Months | 235.6 | 185.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 2.0 | 0.8
12 Months or More | 0.0 | 0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.0 | 0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 235.6 | 185.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 2.0 | 0.8
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] |  |
Schedule of Available-for-sale Securities |  |
Less than 12 Months | 434.0 | 740.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 2.4 | 4.6
12 Months or More | 12.3 | 155.9
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.1 | 1.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 446.3 | 896.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 2.5 | 6.0
State, county and municipal securities |  |
Schedule of Available-for-sale Securities |  |
Less than 12 Months | 148.1 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 1.0 |
12 Months or More | 0.0 |
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.0 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 148.1 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 1.0 |
General obligation securities, amortized cost | 38.6 | 80.1
State, county and municipal securities | Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming |  |
Schedule of Available-for-sale Securities |  |
General obligation securities, amortized cost | 26.0 |
Mortgage-backed Securities, Issued by Private Enterprises [Member] |  |
Schedule of Available-for-sale Securities |  |
Less than 12 Months | 0.0 | 0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0.0 | 0.0
12 Months or More | 4.3 | 46.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.1 | 0.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 4.3 | 46.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 0.1 | $ 0.4
Corporate Debt Securities [Member] |  |
Schedule of Available-for-sale Securities |  |
Less than 12 Months | 20.9 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0.1 |
12 Months or More | 0.0 |
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.0 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 20.9 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ 0.1 |

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- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References No definition available. + Details
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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer. + References No definition available. + Details
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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27337-111563 + Details
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- Definition This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
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