Organization and Basis of Presentation - Summary of Financial Securities (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Held to maturity
Equity and Debt Securities, Amortized Cost	$ 207,402	$ 187,155
Equity and Debt Securities, Gross Unrealized Gain (Loss)	3,194	2,849
Equity and Debt Securities, Marketable Securities	210,596	190,004
Investment funds - debt securities [Member]
Equity securities:
Amortized Cost	31,914	27,914
Gross Unrealized Gain (Loss)	(430)	(29)
Carrying Value	31,484	27,885
Investment funds - U.S. treasuries [Member]
Equity securities:
Amortized Cost	8,929	10,999
Gross Unrealized Gain (Loss)	(90)	(55)
Carrying Value	8,839	10,944
Common stock and stock funds [Member]
Equity securities:
Amortized Cost	39,755	34,329
Gross Unrealized Gain (Loss)	3,835	2,973
Carrying Value	43,590	37,302
Investment-grade unsecured bonds [Member]
Available for sale
Amortized Cost	4,281	4,365
Gross Unrealized Gain (Loss)	(121)	(40)
Carrying Value	4,160	4,325
Mortgage backed securities [Member]
Held to maturity
Amortized Cost	122,523	109,548
Gross Unrealized Gain (Loss)	0	0
Carrying Value	$ 122,523	$ 109,548

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