Investments - Securities and Investments, Recorded at Either Fair Value or Cost (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 228,390	$ 281,363
Gross Unrealized Gains	12,119	15,470
Gross Unrealized Losses	(1,163)	(771)
Available-for-sale-securities, Carrying Value	239,346	296,062
Restricted investment held at cost	30,000	30,000
Other investments held at cost	1,446	1,506
Total carrying value of available-for-sale investments	270,792	327,568
Deferred Compensation Plan Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,046	13,897
Gross Unrealized Gains	2,601	2,409
Gross Unrealized Losses	(113)	(12)
Available-for-sale-securities, Carrying Value	16,534	16,294
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	114,446	111,261
Gross Unrealized Gains	123	186
Gross Unrealized Losses	(139)	(186)
Available-for-sale-securities, Carrying Value	114,430	111,261
Municipal Fixed-Rate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	67,302	127,341
Gross Unrealized Gains	130	480
Gross Unrealized Losses	(45)	(34)
Available-for-sale-securities, Carrying Value	67,387	127,787
Municipal Variable Rate Demand Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,695	2,465
Available-for-sale-securities, Carrying Value	3,695	2,465
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	28,901	26,399
Gross Unrealized Gains	9,265	12,395
Gross Unrealized Losses	(866)	(539)
Available-for-sale-securities, Carrying Value	$ 37,300	$ 38,255