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Organization and Basis of Presentation - Summary of Financial Securities (Details) - USD ($) $ in Thousands | Sep. 30, 2017 | Dec. 31, 2016
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Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities, carrying value | $ 184,574 | $ 139,189
Marketable securities and other investments, amortized cost | 182,784 | 137,715
Marketable securities and other investments, gross unrealized gain (loss) | 1,790 | 1,474
Marketable securities | 184,574 | 139,189
Investment-Grade Unsecured Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Securities, amortized cost | 4,450 |
Available-for-sale securities, gross unrealized gain (loss) | (14) |
Available-for-sale securities, carrying value | 4,436 |
Investment Funds - Debt Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Securities, amortized cost | 28,067 | 19,604
Available-for-sale securities, gross unrealized gain (loss) | 146 | (73)
Available-for-sale securities, carrying value | 28,213 | 19,531
Investment Funds - U.S. Treasuries [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Securities, amortized cost | 10,910 | 10,022
Available-for-sale securities, gross unrealized gain (loss) | (29) | (22)
Available-for-sale securities, carrying value | 10,881 | 10,000
Common Stock and Stock Funds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Securities, amortized cost | 33,816 | 13,696
Available-for-sale securities, gross unrealized gain (loss) | 1,687 | 1,569
Available-for-sale securities, carrying value | 35,503 | 15,265
Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Held-to-maturity Securities, amortized cost | 105,541 | 94,393
Held-to-maturity securities, gross unrealized gain (loss) | 0 | 0
Held-to-maturity securities, carrying value | $ 105,541 | $ 94,393

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