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Summary of Critical and Significant Accounting Policies - Marketable Securities (Details) - USD ($) $ in Thousands | 1 Months Ended | 12 Months Ended
--- | --- | ---
Nov. 30, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Marketable Securities [Line Items] |  |  |  |
Carrying Value |  | $ 2,500 | $ 3,600 |
Equity securities: |  |  |  |
Gross Unrealized Gain |  | 12,515 | 5,710 | $ (5,159)
Carrying Value |  | 2,500 | 3,600 |
Held to maturity: |  |  |  |
Total - Marketable securities, amortized cost |  | 131,770 | 140,649 |
Total - Marketable securities, gross unrealized gain (loss) |  | 15,998 | 3,544 |
Total - Marketable securities, carrying value |  | 147,768 | 144,193 |
Marketable securities, allowance for credit loss |  | 0 | 0 |
Proceeds from maturity of mortgage backed security | $ 83,100 | 91,700 | 83,100 |
Accelerated interest income |  | 11,800 | 7,000 |
Proceeds from sale and maturities of marketable securities |  | 113,500 | 147,500 | 31,500
Marketable securities, realized gain (loss) |  | 2,100 | 1,300 | 700
Unrealized gain (loss) on equity securities recognized through income |  | 12,515 | 5,710 | $ (5,159)
Interest and Other Income |  |  |  |
Equity securities: |  |  |  |
Gross Unrealized Gain |  | 12,500 | 5,700 |
Held to maturity: |  |  |  |
Unrealized gain (loss) on equity securities recognized through income |  | 12,500 | 5,700 |
Investment Funds - Debt Securities |  |  |  |
Marketable Securities [Line Items] |  |  |  |
Carrying Value |  | 50,631 | 30,132 |
Equity securities: |  |  |  |
Amortized Cost |  | 49,646 | 29,588 |
Gross Unrealized Gain |  | 985 | 544 |
Carrying Value |  | 50,631 | 30,132 |
Held to maturity: |  |  |  |
Unrealized gain (loss) on equity securities recognized through income |  | 985 | 544 |
Common Stock and Stock Funds |  |  |  |
Marketable Securities [Line Items] |  |  |  |
Carrying Value |  | 96,075 | 37,868 |
Equity securities: |  |  |  |
Amortized Cost |  | 81,074 | 34,941 |
Gross Unrealized Gain |  | 15,001 | 2,927 |
Carrying Value |  | 96,075 | 37,868 |
Held to maturity: |  |  |  |
Unrealized gain (loss) on equity securities recognized through income |  | 15,001 | 2,927 |
U.S. Treasury Securities |  |  |  |
Available for sale |  |  |  |
Available-for-sale, amortized cost |  |  | 2,421 |
Available-for-sale, gross unrealized gain (Loss) |  |  | 13 |
Available-for-sale, carrying value |  |  | 2,434 |
Investment-Grade Unsecured Debt |  |  |  |
Available for sale |  |  |  |
Available-for-sale, amortized cost |  | 1,050 | 1,048 |
Available-for-sale, gross unrealized gain (Loss) |  | 12 | 60 |
Available-for-sale, carrying value |  | 1,062 | 1,108 |
Allowance for Credit Losses |  | 0 |  |
Mortgage Backed Securities |  |  |  |
Held to maturity: |  |  |  |
Held to maturity, amortized cost |  |  | 72,651 |
Held to maturity, gross unrealized gain (loss) |  |  | 0 |
Held to maturity carrying value |  | $ 0 | $ 72,651 |

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