Organization and Basis of Presentation - Summary of Financial Securities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Held to maturity
Equity and Debt Securities, Amortized Cost	$ 195,850	$ 187,155
Equity and Debt Securities, Gross Unrealized Gain (Loss)	1,895	2,849
Equity and Debt Securities, Marketable Securities	197,745	190,004
Investment Funds - Debt Securities [Member]
Equity securities:
Amortized Cost	26,946	27,914
Gross Unrealized Gain (Loss)	(360)	(29)
Carrying Value	26,586	27,885
Investment Funds - U.S. Treasuries [Member]
Equity securities:
Amortized Cost	11,250	10,999
Gross Unrealized Gain (Loss)	(86)	(55)
Carrying Value	11,164	10,944
Common Stock and Stock Funds [Member]
Equity securities:
Amortized Cost	39,620	34,329
Gross Unrealized Gain (Loss)	2,459	2,973
Carrying Value	42,079	37,302
Investment-Grade Unsecured Bonds [Member]
Available for sale
Amortized Cost	4,324	4,365
Gross Unrealized Gain (Loss)	(118)	(40)
Carrying Value	4,206	4,325
Mortgage Backed Securities [Member]
Held to maturity
Amortized Cost	113,710	109,548
Gross Unrealized Gain (Loss)	0	0
Carrying Value	$ 113,710	$ 109,548