Organization and Basis of Presentation - Summary of Financial Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
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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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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