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Investments - Securities and Investments, Recorded at Either Fair Value or Cost (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 195,549	$ 190,146
Gross Unrealized Gains	4,156	2,709
Gross Unrealized Losses	(1,368)	(2,132)
Available-for-sale-securities, Carrying Value	198,337	190,723
Restricted investment held at cost	27,800	27,800
Other investments held at cost	734	767
Total carrying value of available-for-sale investments	226,871	219,290
Deferred Compensation Plan Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,084	12,367
Gross Unrealized Gains	2,314	2,271
Gross Unrealized Losses	(26)	(42)
Available-for-sale-securities, Carrying Value	17,372	14,596
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	72,324	66,522
Gross Unrealized Gains	99	64
Gross Unrealized Losses	(150)	(174)
Available-for-sale-securities, Carrying Value	72,273	66,412
Municipal Fixed-Rate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,970	11,799
Gross Unrealized Gains	14	12
Gross Unrealized Losses	(25)	(37)
Available-for-sale-securities, Carrying Value	10,959	11,774
Asset-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,832	10,201
Gross Unrealized Gains	10	19
Gross Unrealized Losses	(14)	(14)
Available-for-sale-securities, Carrying Value	11,828	10,206
Mortgage/Agency-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,370	13,080
Gross Unrealized Gains	23	15
Gross Unrealized Losses	(80)	(91)
Available-for-sale-securities, Carrying Value	12,313	13,004
U.S. Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	28,251	30,022
Gross Unrealized Gains	27	15
Gross Unrealized Losses	(231)	(270)
Available-for-sale-securities, Carrying Value	28,047	29,767
Foreign Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,279	3,729
Gross Unrealized Gains		2
Gross Unrealized Losses		(1)
Available-for-sale-securities, Carrying Value	3,279	3,730
Variable Rate Demand Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,505	11,855
Available-for-sale-securities, Carrying Value	10,505	11,855
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30,934	30,571
Gross Unrealized Gains	1,669	311
Gross Unrealized Losses	(842)	(1,503)
Available-for-sale-securities, Carrying Value	$ 31,761	$ 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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