Marketable Securities (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Marketable Securities, Realized Gain (Loss)	$ 0	$ 0
Realized Gain (Loss) on Marketable Securities and Cost Method Investments, Excluding Other than Temporary Impairments, and Other Investments		0
Amortized Cost Basis	1,468,798,000		$ 1,380,487,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	9,827,000		6,241,000
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(15,619,000)		(18,987,000)
Available-for-sale Securities	1,463,006,000		1,367,741,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year	495,311,000		503,482,000
Maturities after one year through five years	898,045,000		815,040,000
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value	15,000,000		0
Total	1,408,356,000		1,318,522,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	894,533,000		883,950,000
Fair Value - 12 Months or Greater	17,632,000		36,407,000
Fair Value - Total	912,165,000		920,357,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(15,592,000)		(18,929,000)
Unrealized Loss - 12 Months or Greater	(27,000)		(58,000)
Unrealized Loss - Total	(15,619,000)		(18,987,000)
Realized Gain (Loss) on Marketable Securities, Cost Method Investments, and Other Investments	0
Realized Gain (Loss) on Marketable Securities, Cost Method Investments, and Other Investments		$ 0
Corporate Bond Securities [Member]
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	740,821,000		759,222,000
Fair Value - 12 Months or Greater	17,632,000		36,407,000
Fair Value - Total	758,453,000		795,629,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(3,736,000)		(4,685,000)
Unrealized Loss - 12 Months or Greater	(27,000)		(58,000)
Unrealized Loss - Total	(3,763,000)		(4,743,000)
U.S. government and government agency obligations
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	105,566,000		81,170,000
Fair Value - 12 Months or Greater	0		0
Fair Value - Total	105,566,000		81,170,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(618,000)		(641,000)
Unrealized Loss - 12 Months or Greater	0		0
Unrealized Loss - Total	(618,000)		(641,000)
Municipal bonds
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	4,123,000		7,141,000
Fair Value - 12 Months or Greater	0		0
Fair Value - Total	4,123,000		7,141,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(15,000)		(20,000)
Unrealized Loss - 12 Months or Greater	0		0
Unrealized Loss - Total	(15,000)		(20,000)
Equity Securities
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	44,023,000		36,417,000
Fair Value - 12 Months or Greater	0		0
Fair Value - Total	44,023,000		36,417,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(11,223,000)		(13,583,000)
Unrealized Loss - 12 Months or Greater	0		0
Unrealized Loss - Total	(11,223,000)		(13,583,000)
Unrestricted [Member] | Corporate Bond Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	1,208,562,000		1,076,964,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,175,000		630,000
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(3,763,000)		(4,743,000)
Available-for-sale Securities	1,205,974,000		1,072,851,000
Unrestricted [Member] | U.S. government and government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	126,082,000		132,923,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	28,000		58,000
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(618,000)		(641,000)
Available-for-sale Securities	125,492,000		132,340,000
Unrestricted [Member] | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	4,638,000		7,663,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2,000		1,000
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(15,000)		(20,000)
Available-for-sale Securities	4,625,000		7,644,000
Unrestricted [Member] | Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	32,620,000		63,074,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0		1,000
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0		0
Available-for-sale Securities	32,620,000		63,075,000
Unrestricted [Member] | Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	39,645,000		42,612,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0		0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0		0
Available-for-sale Securities	39,645,000		42,612,000
Unrestricted [Member] | Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	57,251,000		57,251,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	8,622,000		5,551,000
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(11,223,000)		(13,583,000)
Available-for-sale Securities	$ 54,650,000		$ 49,219,000