Organization and Basis of Presentation - Summary of Financial Securities (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Held to maturity
Equity and Debt Securities, Amortized Cost	$ 205,193	$ 187,155
Equity and Debt Securities, Gross Unrealized Gain (Loss)	1,998	2,849
Equity and Debt Securities, Marketable Securities	207,191	190,004
Investment funds - debt securities [Member]
Equity securities:
Amortized Cost	31,852	27,914
Gross Unrealized Gain (Loss)	(371)	(29)
Carrying Value	31,481	27,885
Investment funds - U.S. treasuries [Member]
Equity securities:
Amortized Cost	11,250	10,999
Gross Unrealized Gain (Loss)	(85)	(55)
Carrying Value	11,165	10,944
Common stock and stock funds [Member]
Equity securities:
Amortized Cost	39,680	34,329
Gross Unrealized Gain (Loss)	2,591	2,973
Carrying Value	42,271	37,302
Investment-grade unsecured bonds [Member]
Available for sale
Amortized Cost	4,379	4,365
Gross Unrealized Gain (Loss)	(137)	(40)
Carrying Value	4,242	4,325
Mortgage backed securities [Member]
Held to maturity
Amortized Cost	118,032	109,548
Gross Unrealized Gain (Loss)	0	0
Carrying Value	$ 118,032	$ 109,548

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