Investments - Securities and Investments, Recorded at Either Fair Value or Cost (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 369,400	$ 426,467
Gross Unrealized Gains	15,826	17,081
Gross Unrealized Losses	(467)	(490)
Available-for-sale-securities, carrying value	384,759	443,058
Restricted investment held at cost	48,250	48,250
Other investments held at cost	1,753	1,902
Total carrying value of available-for-sale investments	434,762	493,210
Deferred Compensation Plan Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,308	10,688
Gross Unrealized Gains	2,490	846
Gross Unrealized Losses	(18)	(7)
Available-for-sale-securities, carrying value	13,780	11,527
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	177,798	185,464
Gross Unrealized Gains	446	966
Gross Unrealized Losses	(103)	(18)
Available-for-sale-securities, carrying value	178,141	186,412
Municipal Fixed-Rate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	145,509	174,530
Gross Unrealized Gains	497	627
Gross Unrealized Losses	(71)	(73)
Available-for-sale-securities, carrying value	145,935	175,084
Municipal Variable Rate Demand Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,960	34,375
Available-for-sale-securities, carrying value	11,960	34,375
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,825	20,966
Gross Unrealized Gains	12,393	14,630
Gross Unrealized Losses	(275)	(392)
Available-for-sale-securities, carrying value	34,943	35,204
Fixed Income Bond Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		444
Gross Unrealized Gains		12
Available-for-sale-securities, carrying value		$ 456