Summary of Critical and Significant Accounting Policies - Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
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

Marketable Securities, Accumulated Gross Unrealized Gain (Loss), Before Tax

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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