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Investment Securities | 12 Months Ended
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Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract] |
Investment Securities | INVESTMENT SECURITIES The amortized cost and approximate fair values of investment securities are summarized as follows:
December 31, 2020 | Amortized Cost | Gross Unrealized Gains | Gross Unrealized Losses | Estimated Fair Value
Available-for-Sale |  |  |  |
State, county, and municipal securities | $ | 462.1 |  | $ | 4.8 |  | $ | (1.0) |  | $ | 465.9 |
Obligations of U.S. government agencies | 332.9 |  | 1.0 |  | (2.0) |  | 331.9 |
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations | 2,830.8 |  | 69.3 |  | (2.5) |  | 2,897.6 |
Private mortgage-backed securities | 10.9 |  | 0.1 |  | (0.1) |  | 10.9 |
Corporate Securities | 295.8 |  | 6.5 |  | (0.1) |  | 302.2 |
Other investments | 0.2 |  | — |  | — |  | 0.2 |
Total | $ | 3,932.7 |  | $ | 81.7 |  | $ | (5.7) |  | $ | 4,008.7 |
December 31, 2020 | Amortized Cost | Gross Unrealized Gains | Gross Unrealized Losses | Estimated Fair Value
Held-to Maturity |  |  |  |
State, county, and municipal securities | $ | 46.6 |  | $ | 3.2 |  | $ | — |  | $ | 49.8 |
U.S agency residential mortgage-backed securities & collateralized mortgage obligations | 1.0 |  | 0.1 |  | — |  | 1.1 |
Corporate securities | 3.9 |  | 0.1 |  | — |  | 4.0 |
Other investments | 0.1 |  | — |  | — |  | 0.1 |
Total | $ | 51.6 |  | $ | 3.4 |  | $ | — |  | $ | 55.0 |
December 31, 2019 | Amortized Cost | Gross Unrealized Gains | Gross Unrealized Losses | Estimated Fair Value
Available-for-Sale |  |  |  |
U.S. Treasury notes | $ | 9.0 |  | $ | — |  | $ | — |  | $ | 9.0 |
State, county, and municipal securities | 80.1 |  | 0.8 |  | — |  | 80.9 |
Obligations of U.S. government agencies | 367.5 |  | 0.1 |  | (0.8) |  | 366.8 |
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations | 2,303.6 |  | 19.6 |  | (6.0) |  | 2,317.2 |
Private mortgage-backed securities | 47.6 |  | — |  | (0.4) |  | 47.2 |
Corporate Securities | 134.5 |  | 1.2 |  | — |  | 135.7 |
Other investments | 3.2 |  | — |  | — |  | 3.2 |
Total | $ | 2,945.5 |  | $ | 21.7 |  | $ | (7.2) |  | $ | 2,960.0 |
December 31, 2019 | Amortized Cost | Gross Unrealized Gains | Gross Unrealized Losses | Estimated Fair Value
Held-to Maturity |  |  |  |
State, county, and municipal securities | $ | 57.3 |  | $ | 2.1 |  | $ | — |  | $ | 59.4 |
Obligations of U.S. government agencies | 19.8 |  | — |  | — |  | 19.8 |
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations | 1.2 |  | — |  | — |  | 1.2 |
Corporate securities | 13.9 |  | 0.1 |  | — |  | 14.0 |
Other investments | 0.1 |  | — |  | — |  | 0.1 |
Total | $ | 92.3 |  | $ | 2.2 |  | $ | — |  | $ | 94.5 |
There were no material gross realized gains and no material gross realized losses on the disposition of available-for-sale securities during 2020, 2019, or 2018. As of December 31, 2020, the Company had general obligation securities with amortized costs of $38.6 million included in state, county, and municipal securities, of which $26.0 million, or 67.4% were issued by political subdivisions or agencies within the states of Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming. The following tables show the gross unrealized losses and fair values of investment securities, aggregated by investment category, and the length of time individual investment securities have been in a continuous unrealized loss position, as of December 31, 2020 and 2019. There were no held-to-maturity securities in a continuous unrealized loss position as of December 31, 2020.
| Less than 12 Months | 12 Months or More | Total
December 31, 2020 | Fair Value | Gross Unrealized Losses | Fair Value | Gross Unrealized Losses | Fair Value | Gross Unrealized Losses
Available-for-Sale |  |  |  |  |  |
State, county, and municipal securities | $ | 148.1 |  | $ | (1.0) |  | $ | — |  | $ | — |  | $ | 148.1 |  | $ | (1.0) |
Obligations of U.S. government agencies | 235.6 |  | (2.0) |  | — |  | — |  | 235.6 |  | (2.0) |
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations | 434.0 |  | (2.4) |  | 12.3 |  | (0.1) |  | 446.3 |  | (2.5) |
Private mortgage-backed securities | — |  | — |  | 4.3 |  | (0.1) |  | 4.3 |  | (0.1) |
Corporate securities | 20.9 |  | (0.1) |  | — |  | — |  | 20.9 |  | (0.1) |
Total | $ | 838.6 |  | $ | (5.5) |  | $ | 16.6 |  | $ | (0.2) |  | $ | 855.2 |  | $ | (5.7) |
| Less than 12 Months | 12 Months or More | Total
December 31, 2019 | Fair Value | Gross Unrealized Losses | Fair Value | Gross Unrealized Losses | Fair Value | Gross Unrealized Losses
Available-for-Sale |  |  |  |  |  |
Obligations of U.S. government agencies | $ | 185.3 |  | $ | (0.8) |  | $ | — |  | $ | — |  | $ | 185.3 |  | $ | (0.8) |
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations | 740.1 |  | (4.6) |  | 155.9 |  | (1.4) |  | 896.0 |  | (6.0) |
Private mortgage-backed securities | — |  | — |  | 46.6 |  | (0.4) |  | 46.6 |  | (0.4) |
Total | $ | 925.4 |  | $ | (5.4) |  | $ | 202.5 |  | $ | (1.8) |  | $ | 1,127.9 |  | $ | (7.2) |
The available-for-sale securities portfolio contains securities that are guaranteed by a sovereign entity or are generally considered to have non-credit related risks, such as interest rate risk or prepayment and liquidity factors. The Company considers whether the securities are issued by the federal government or its agencies and whether downgrades by bond rating agencies have occurred. The unrealized losses are due to changes in interest rates and other market conditions. The Company had 181 and 338 individual investment securities that were in an unrealized loss position as of December 31, 2020 and 2019, respectively, related primarily to fluctuations in the current interest rates. As of December 31, 2020, the Company had the intent and ability to hold these investment securities for a period of time sufficient to allow for an anticipated recovery. Furthermore, the Company does not have the intent to sell any of the available-for-sale securities in the above table and it is more likely than not that the Company will not have to sell any securities before a recovery in cost. There were no material allowances for credit loss as of December 31, 2020 and no impairment losses were recorded during 2019 or 2018 for investment securities. Maturities of securities do not reflect rate repricing opportunities present in adjustable-rate mortgage-backed securities. As of December 31, 2020 and 2019, the Company had variable rate mortgage-backed securities with amortized costs of $220.7 million and $298.1 million, respectively, classified as available-for-sale in the table below. Maturities of investment securities as of December 31, 2020 are shown below. Maturities of mortgage-backed securities have been adjusted to reflect shorter maturities based upon estimated prepayments of principal. All other investment securities maturities are shown at contractual maturity dates.
| Available-for-Sale |  | Held-to-Maturity
December 31, 2020 | Amortized Cost | Estimated Fair Value |  | Amortized Cost | Estimated Fair Value
Within one year | $ | 687.6 |  | $ | 991.4 |  |  | $ | 8.3 |  | $ | 8.4 |
After one year but within five years | 1,587.7 |  | 1,362.6 |  |  | 27.8 |  | 29.0 |
After five years but within ten years | 736.2 |  | 996.3 |  |  | 14.1 |  | 16.2 |
After ten years | 921.2 |  | 658.4 |  |  | 1.4 |  | 1.4 |
Total | $ | 3,932.7 |  | $ | 4,008.7 |  |  | $ | 51.6 |  | $ | 55.0 |
As of December 31, 2020, the Company had investment securities callable within one year with amortized costs and estimated fair values of $282.4 million and $281.7 million, respectively. These investment securities are primarily classified as available-for-sale and included in the after five years but within ten years category in the table above. As of December 31, 2020, the Company had no callable structured notes. There were no significant concentrations of investments at December 31, 2020, (greater than 10 percent of stockholders’ equity) in any individual security issuer, except for U.S. government or agency-backed securities. As of December 31, 2020 and 2019, the Company recorded amortized costs of $2,323.0 million and $2,132.0 million, respectively, for investment securities pledged to secure public deposits and securities sold under repurchase agreements and had approximate fair values as of December 31, 2020 and 2019 of $2,383.6 million and $2,144.9 million, respectively. All securities sold under repurchase agreements are with clients and mature on the next banking day. The Company retains possession of the underlying securities sold under repurchase agreements.

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- Definition The entire disclosure for investments in certain debt and equity securities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2209399 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2324412 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -URI http://asc.fasb.org/topic&trid=2196928 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2176304 + Details
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