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Investments - Securities and Investments, Recorded at Either Fair Value or Cost (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 114,139	$ 190,146
Gross Unrealized Gains	5,262	2,709
Gross Unrealized Losses	(1,363)	(2,132)
Available-for-sale-securities, Fair Value/Carrying Value	118,038	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	146,385	219,290
Deferred Compensation Plan Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	17,804	12,367
Gross Unrealized Gains	2,175	2,271
Gross Unrealized Losses	(96)	(42)
Available-for-sale-securities, Fair Value/Carrying Value	19,883	14,596
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	32,654	66,522
Gross Unrealized Gains	44	64
Gross Unrealized Losses	(155)	(174)
Available-for-sale-securities, Fair Value/Carrying Value	32,543	66,412
Municipal Fixed-Rate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,902	11,799
Gross Unrealized Gains	2	12
Gross Unrealized Losses	(22)	(37)
Available-for-sale-securities, Fair Value/Carrying Value	2,882	11,774
Asset-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,545	10,201
Gross Unrealized Gains	1	19
Gross Unrealized Losses	(20)	(14)
Available-for-sale-securities, Fair Value/Carrying Value	6,526	10,206
Mortgage/Agency-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,554	13,080
Gross Unrealized Gains	1	15
Gross Unrealized Losses	(46)	(91)
Available-for-sale-securities, Fair Value/Carrying Value	5,509	13,004
U.S. Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,477	30,022
Gross Unrealized Gains		15
Gross Unrealized Losses	(174)	(270)
Available-for-sale-securities, Fair Value/Carrying Value	14,303	29,767
Foreign Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	725	3,729
Gross Unrealized Gains	5	2
Gross Unrealized Losses		(1)
Available-for-sale-securities, Fair Value/Carrying Value	730	3,730
Variable Rate Demand Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		11,855
Available-for-sale-securities, Fair Value/Carrying Value		11,855
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	33,478	30,571
Gross Unrealized Gains	3,034	311
Gross Unrealized Losses	(850)	(1,503)
Available-for-sale-securities, Fair Value/Carrying Value	$ 35,662	$ 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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