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Investment Securities - Realized Gains (Losses) on Investments (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Schedule of Available-for-sale Securities |  |  |
Debt Securities, Available-for-sale, Amortized Cost | $ 4,859.3 | $ 3,932.7 |
Realized Investment Gains (Losses) |  | 0.0 | $ 0.0
Debt Securities, Available-for-sale, Realized Gain | 3.2 |  |
Debt Securities, Available-for-sale, Realized Loss | 2.1 |  |
US Government Agencies Debt Securities [Member] |  |  |
Schedule of Available-for-sale Securities |  |  |
Debt Securities, Available-for-sale, Amortized Cost | 356.0 | 332.9 |
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] |  |  |
Schedule of Available-for-sale Securities |  |  |
Debt Securities, Available-for-sale, Amortized Cost | 2,027.3 | 2,830.8 |
State, county and municipal securities |  |  |
Schedule of Available-for-sale Securities |  |  |
Debt Securities, Available-for-sale, Amortized Cost | 434.7 | 462.1 |
State, county and municipal securities | Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming |  |  |
Schedule of Available-for-sale Securities |  |  |
Debt Securities, Available-for-sale, Amortized Cost | 40.7 |  |
Mortgage-backed Securities, Issued by Private Enterprises [Member] |  |  |
Schedule of Available-for-sale Securities |  |  |
Debt Securities, Available-for-sale, Amortized Cost | 174.4 | 10.9 |
Corporate Debt Securities [Member] |  |  |
Schedule of Available-for-sale Securities |  |  |
Debt Securities, Available-for-sale, Amortized Cost | 271.1 | 295.8 |
Other Debt Obligations [Member] |  |  |
Schedule of Available-for-sale Securities |  |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 0.2 |
Mortgage Backed Securities, Variable Rate |  |  |
Schedule of Available-for-sale Securities |  |  |
Debt Securities, Available-for-sale, Amortized Cost | $ 84.8 | $ 220.7 |

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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). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -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/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454 + Details
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- Definition Amount of realized gain 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://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27357-111563 + Details
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- Definition Amount of realized loss 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://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27357-111563 + Details
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