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Investments - Securities and Investments, Recorded at Either Fair Value or Cost (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 175,043	$ 190,146
Gross Unrealized Gains	4,713	2,709
Gross Unrealized Losses	(1,264)	(2,132)
Available-for-sale-securities, Carrying Value	178,492	190,723
Restricted investment held at cost	27,800	27,800
Other investments held at cost	673	767
Total carrying value of available-for-sale investments	206,965	219,290
Deferred Compensation Plan Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,662	12,367
Gross Unrealized Gains	2,389	2,271
Gross Unrealized Losses	(23)	(42)
Available-for-sale-securities, Carrying Value	18,028	14,596
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	72,954	66,522
Gross Unrealized Gains	132	64
Gross Unrealized Losses	(107)	(174)
Available-for-sale-securities, Carrying Value	72,979	66,412
Municipal Fixed-Rate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,596	11,799
Gross Unrealized Gains	7	12
Gross Unrealized Losses	(20)	(37)
Available-for-sale-securities, Carrying Value	7,583	11,774
Asset-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,313	10,201
Gross Unrealized Gains	11	19
Gross Unrealized Losses	(16)	(14)
Available-for-sale-securities, Carrying Value	13,308	10,206
Mortgage/Agency-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,549	13,080
Gross Unrealized Gains	14	15
Gross Unrealized Losses	(36)	(91)
Available-for-sale-securities, Carrying Value	9,527	13,004
U.S. Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,528	30,022
Gross Unrealized Gains	4	15
Gross Unrealized Losses	(132)	(270)
Available-for-sale-securities, Carrying Value	21,400	29,767
Foreign Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,700	3,729
Gross Unrealized Gains	3	2
Gross Unrealized Losses	(1)	(1)
Available-for-sale-securities, Carrying Value	2,702	3,730
Variable Rate Demand Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		11,855
Available-for-sale-securities, Carrying Value		11,855
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	31,741	30,571
Gross Unrealized Gains	2,153	311
Gross Unrealized Losses	(929)	(1,503)
Available-for-sale-securities, Carrying Value	$ 32,965	$ 29,379

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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