Fair Value of Financial Instruments (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Fair value of assets and liabilities measured on recurring basis
Marketable securities, current	$ 160,442,000		$ 137,127,000
Marketable securities, noncurrent	319,150,000		318,355,000
Other-than-temporary impairment of available-for-sale securities	0
Proceeds from the sales and maturities of available-for-sale securities	75,000,000	178,000,000
Minimum
Fair value of assets and liabilities measured on recurring basis
Maturity of marketable securities, noncurrent	1 year
Maximum
Fair value of assets and liabilities measured on recurring basis
Maturity of marketable securities, noncurrent	5 years
Fair Value, Measurements, Recurring | Money market funds
Fair value of assets and liabilities measured on recurring basis
Available-for-sale securities	11,000,000		14,000,000
Fair Value, Measurements, Recurring | U.S. agency securities
Fair value of assets and liabilities measured on recurring basis
Available-for-sale securities	165,000,000		161,000,000
Fair Value, Measurements, Recurring | U.S. Treasury securities
Fair value of assets and liabilities measured on recurring basis
Available-for-sale securities	65,000,000		67,000,000
Fair Value, Measurements, Recurring | Corporate debt securities
Fair value of assets and liabilities measured on recurring basis
Available-for-sale securities	186,000,000		184,000,000
Fair Value, Measurements, Recurring | Other securities
Fair value of assets and liabilities measured on recurring basis
Available-for-sale securities	9,000,000		9,000,000
Fair Value, Measurements, Recurring | Total
Fair value of assets and liabilities measured on recurring basis
Cash and cash equivalents	11,245,000		14,457,000
Marketable securities, current	105,375,000		102,439,000
Deferred compensation trusts	93,173,000		80,842,000
Marketable securities, noncurrent	319,150,000		318,355,000
Fair Value, Measurements, Recurring | Total | Commodity Swap Forward Contracts
Fair value of assets and liabilities measured on recurring basis
Derivative assets	53,000		95,000
Derivative liabilities	1,000		28,000
Fair Value, Measurements, Recurring | Total | Foreign currency contracts
Fair value of assets and liabilities measured on recurring basis
Derivative assets	3,404,000		640,000
Derivative liabilities	328,000		2,151,000
Fair Value, Measurements, Recurring | Level 1
Fair value of assets and liabilities measured on recurring basis
Cash and cash equivalents	11,245,000		14,457,000
Deferred compensation trusts	93,173,000		80,842,000
Fair Value, Measurements, Recurring | Level 2
Fair value of assets and liabilities measured on recurring basis
Marketable securities, current	105,375,000		102,439,000
Marketable securities, noncurrent	319,150,000		318,355,000
Fair Value, Measurements, Recurring | Level 2 | Commodity Swap Forward Contracts
Fair value of assets and liabilities measured on recurring basis
Derivative assets	53,000		95,000
Derivative liabilities	1,000		28,000
Fair Value, Measurements, Recurring | Level 2 | Foreign currency contracts
Fair value of assets and liabilities measured on recurring basis
Derivative assets	3,404,000		640,000
Derivative liabilities	$ 328,000		$ 2,151,000