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Summary of Critical and Significant Accounting Policies - Marketable Securities (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract] |  |  |
Total - Marketable securities, Amortized Cost | $ 211,960 | $ 187,155 |
Total - Marketable securities, Gross Unrealized Gain (Loss) | (2,415) | 2,849 |
Total - Marketable securities, Carrying Value | 209,545 | 190,004 |
Proceeds from sale and maturities of marketable securities | 31,500 | 35,500 | $ 30,500
Marketable securities, gains | 700 | 1,900 | $ 5,700
Interest and Other Income [Member] |  |  |
Debt Securities, Available-for-sale [Abstract] |  |  |
Equity Securities, Gross Unrealized Gain (Loss) | 5,200 |  |
Investment funds - debt securities [Member] |  |  |
Debt Securities, Available-for-sale [Abstract] |  |  |
Equity Securities, Amortized cost | 31,934 | 27,914 |
Equity Securities, Gross Unrealized Gain (Loss) | 568 | 29 |
Equity Securities, Carrying Value | 31,366 | 27,885 |
Investment funds - U.S. treasuries [Member] |  |  |
Debt Securities, Available-for-sale [Abstract] |  |  |
Equity Securities, Amortized cost | 8,983 | 10,999 |
Equity Securities, Gross Unrealized Gain (Loss) | 31 | 55 |
Equity Securities, Carrying Value | 8,952 | 10,944 |
Common stock and stock funds [Member] |  |  |
Debt Securities, Available-for-sale [Abstract] |  |  |
Equity Securities, Amortized cost | 39,731 | 34,329 |
Equity Securities, Gross Unrealized Gain (Loss) | 1,671 | (2,973) |
Equity Securities, Carrying Value | 38,060 | 37,302 |
Investment-grade unsecured bonds [Member] |  |  |
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract] |  |  |
Debt Securities, Available-for-sale, Amortized Cost | 4,125 | 4,365 |
Debt Securities, Available-for-sale, Gross Unrealized Gain (Loss) | (145) | (40) |
Debt Securities, Available-for-sale, Carrying Value | 3,980 | 4,325 |
Mortgage backed securities [Member] |  |  |
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract] |  |  |
Held to maturity, Amortized Cost | 127,187 | 109,548 |
Held to maturity, Gross Unrealized Gain (Loss) | 0 | 0 |
Held to maturity, Carrying Value | $ 127,187 | $ 109,548 |

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