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Investment Securities | 9 Months Ended
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Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract] |
Investment Securities | Investment Securities The amortized cost and the approximate fair values of investment securities are summarized as follows:
September 30, 2021 | Amortized Cost | Gross Unrealized Gains | Gross Unrealized Losses |  | Estimated Fair Value
Available-for-Sale: |  |  |  |  |
U.S. Treasury notes | $ | 697.4 |  | $ | — |  | $ | (5.6) |  |  | $ | 691.8 |
State, county, and municipal securities | 444.9 |  | 2.1 |  | (7.1) |  |  | 439.9 |
Obligations of U.S. government agencies | 332.3 |  | 0.2 |  | (6.6) |  |  | 325.9 |
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations | 2,021.4 |  | 25.9 |  | (16.0) |  |  | 2,031.3 |
Private mortgage-backed securities | 83.7 |  | 0.1 |  | (0.3) |  |  | 83.5 |
Collateralized loan obligations | 452.4 |  | 0.6 |  | — |  |  | 453.0 |
Corporate securities | 376.7 |  | 4.2 |  | (2.7) |  |  | 378.2 |
Total | $ | 4,408.8 |  | $ | 33.1 |  | $ | (38.3) |  |  | $ | 4,403.6 |
September 30, 2021 | Amortized Cost | Gross Unrealized Gains | Gross Unrealized Losses | Estimated Fair Value |
Held-to-Maturity: |  |  |  |  |
State, county, and municipal securities | $ | 69.7 |  | $ | 2.3 |  | $ | (0.5) |  | $ | 71.5 |  |
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations (1) | 1,537.5 |  | 20.5 |  | (8.4) |  | 1,549.6 |  |
Corporate securities | 10.9 |  | 0.5 |  | — |  | 11.4 |  |
Total | $ | 1,618.1 |  | $ | 23.3 |  | $ | (8.9) |  | $ | 1,632.5 |  |
(1) Amortized costs presented above include $22.3 million of unamortized gains in U.S. agency residential mortgage-backed securities and collateralized mortgage obligations related to the 2021 second quarter transfer of securities from available-for-sale to held-to-maturity. |
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 |  |
There were no material gross realized gains and no material gross losses on the disposition of available-for-sale investment securities during the three and nine month periods ended September 30, 2021 and 2020. As of September 30, 2021, the Company had general obligation securities with amortized costs of $56.9 million included in state, county, and municipal securities, of which $40.9 million, or 71.9%, 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 an unrealized loss position as of September 30, 2021 and December 31, 2020. There were no held-to-maturity securities in an unrealized loss position at December 31, 2020.
| Less than 12 Months |  | 12 Months or More |  | Total
September 30, 2021 | Fair Value | Gross Unrealized Losses |  | Fair Value | Gross Unrealized Losses |  | Fair Value | Gross Unrealized Losses
Available-for-Sale: |  |  |  |  |  |  |  |
U.S. Treasury notes | $ | 691.8 |  | $ | (5.6) |  |  | $ | — |  | $ | — |  |  | $ | 691.8 |  | $ | (5.6) |
State, county, and municipal securities | 287.7 |  | (7.1) |  |  | — |  | — |  |  | 287.7 |  | (7.1) |
Obligations of U.S. government agencies | 301.2 |  | (6.6) |  |  | — |  | — |  |  | 301.2 |  | (6.6) |
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations | 964.9 |  | (16.0) |  |  | — |  | — |  |  | 964.9 |  | (16.0) |
Private mortgage-backed securities | 54.3 |  | (0.3) |  |  | — |  | — |  |  | 54.3 |  | (0.3) |
Corporate securities | 158.7 |  | (2.6) |  |  | 21.2 |  | (0.1) |  |  | 179.9 |  | (2.7) |
Total | $ | 2,458.6 |  | $ | (38.2) |  |  | $ | 21.2 |  | $ | (0.1) |  |  | $ | 2,479.8 |  | $ | (38.3) |
| Less than 12 Months |  | 12 Months or More |  | Total
September 30, 2021 | Fair Value | Gross Unrealized Losses |  | Fair Value | Gross Unrealized Losses |  | Fair Value | Gross Unrealized Losses
Held-to-Maturity: |  |  |  |  |  |  |  |
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations | $ | 772.3 |  | $ | (8.4) |  |  | $ | — |  | $ | — |  |  | $ | 772.3 |  | $ | (8.4) |
State, county, and municipal securities | 29.0 |  | (0.5) |  |  | — |  | — |  |  | 29.0 |  | (0.5) |
Total | $ | 801.3 |  | $ | (8.9) |  |  | $ | — |  | $ | — |  |  | $ | 801.3 |  | $ | (8.9) |
| 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) |
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. As of September 30, 2021 and December 31, 2020, the Company had 278 and 181 individual investment securities, respectively, that were in an unrealized loss position, related primarily to fluctuations in current interest rates. As of September 30, 2021, 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 intend to sell any of the available-for-sale securities in the above table and the Company does not anticipate it will have to sell any securities before a recovery in cost. There were no material allowances for credit loss as of September 30, 2021 or December 31, 2020 for available-for-sale or held-to-maturity securities. Maturities of securities do not reflect rate repricing opportunities present in adjustable-rate mortgage-backed securities. In the table below, the Company had variable rate mortgage-backed and corporate securities which had an amortized cost of $540.2 million and were classified as available-for-sale as of September 30, 2021. 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
September 30, 2021 | Amortized Cost | Estimated Fair Value |  | Amortized Cost | Estimated Fair Value
Within one year | $ | 625.0 |  | $ | 722.7 |  |  | $ | 662.2 |  | $ | 667.4 |
After one year but within five years | 1,514.8 |  | 1,621.1 |  |  | 365.7 |  | 369.7 |
After five years but within ten years | 1,295.3 |  | 1,161.6 |  |  | 263.6 |  | 266.8 |
After ten years | 973.7 |  | 898.2 |  |  | 326.6 |  | 328.6 |
Total | $ | 4,408.8 |  | $ | 4,403.6 |  |  | $ | 1,618.1 |  | $ | 1,632.5 |
As of September 30, 2021, the Company held investment securities callable within one year with amortized costs and estimated fair values of $212.3 million and $209.9 million, respectively. These investment securities are primarily included in the “after five years but within ten years” category in the table above. As of September 30, 2021, the Company held no callable structured notes. As of September 30, 2021 and December 31, 2020, the Company recorded amortized costs of $2,373.4 million and $2,323.0 million, respectively, for investment securities pledged to secure public deposits and securities sold under repurchase agreements and had approximate fair values as of September 30, 2021 and December 31, 2020 of $2,399.1 million and $2,383.6 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 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher SEC -Name Regulation S-K (SK) -Number 229 -Section 1403 -Paragraph (b) + Details
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