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Investments - Securities and Investments, Recorded at Either Fair Value or Cost (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 135,282	$ 190,146
Gross Unrealized Gains	5,976	2,709
Gross Unrealized Losses	(1,233)	(2,132)
Available-for-sale-securities, Carrying Value	140,025	190,723
Restricted investment held at cost	27,800	27,800
Other investments held at cost	547	767
Total carrying value of available-for-sale investments	168,372	219,290
Deferred Compensation Plan Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,898	12,367
Gross Unrealized Gains	3,230	2,271
Gross Unrealized Losses	(15)	(42)
Available-for-sale-securities, Carrying Value	19,113	14,596
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	45,223	66,522
Gross Unrealized Gains	75	64
Gross Unrealized Losses	(79)	(174)
Available-for-sale-securities, Carrying Value	45,219	66,412
Municipal Fixed-Rate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,887	11,799
Gross Unrealized Gains	4	12
Gross Unrealized Losses	(17)	(37)
Available-for-sale-securities, Carrying Value	4,874	11,774
Asset-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,791	10,201
Gross Unrealized Gains	5	19
Gross Unrealized Losses	(11)	(14)
Available-for-sale-securities, Carrying Value	7,785	10,206
Mortgage/Agency-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,364	13,080
Gross Unrealized Gains	7	15
Gross Unrealized Losses	(41)	(91)
Available-for-sale-securities, Carrying Value	7,330	13,004
U.S. Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,000	30,022
Gross Unrealized Gains	3	15
Gross Unrealized Losses	(109)	(270)
Available-for-sale-securities, Carrying Value	20,894	29,767
Foreign Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	725	3,729
Gross Unrealized Gains	3	2
Gross Unrealized Losses		(1)
Available-for-sale-securities, Carrying Value	728	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	32,394	30,571
Gross Unrealized Gains	2,649	311
Gross Unrealized Losses	(961)	(1,503)
Available-for-sale-securities, Carrying Value	$ 34,082	$ 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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