Marketable Securities (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Document Period End Date	Mar. 31, 2018
Marketable Securities, Realized Gain (Loss)	$ 0	$ 0
Unrealized Gain on Securities	9,400,000	$ 5,400,000
Amortized Cost Basis	2,376,174,000		$ 2,027,272,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,978,000		2,210,000
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(19,781,000)		(8,926,000)
Available-for-sale Securities	2,358,371,000		2,020,556,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year	605,461,000		593,783,000
Maturities after one year through five years	1,752,910,000		1,426,773,000
Total	2,358,371,000		2,020,556,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	1,417,229,000		1,044,084,000
Fair Value - 12 Months or Greater	311,051,000		326,676,000
Fair Value - Total	1,728,280,000		1,370,760,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(14,584,000)		(5,343,000)
Unrealized Loss - 12 Months or Greater	(5,197,000)		(3,583,000)
Unrealized Loss - Total	(19,781,000)		(8,926,000)
Corporate Bond Securities [Member]
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	1,343,778,000		930,970,000
Fair Value - 12 Months or Greater	234,258,000		256,750,000
Fair Value - Total	1,578,036,000		1,187,720,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(13,950,000)		(4,924,000)
Unrealized Loss - 12 Months or Greater	(4,209,000)		(2,748,000)
Unrealized Loss - Total	(18,159,000)		(7,672,000)
U.S. government and government agency obligations
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	70,872,000		110,532,000
Fair Value - 12 Months or Greater	76,793,000		67,921,000
Fair Value - Total	147,665,000		178,453,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(623,000)		(409,000)
Unrealized Loss - 12 Months or Greater	(988,000)		(832,000)
Unrealized Loss - Total	(1,611,000)		(1,241,000)
Municipal bonds
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	2,579,000		2,582,000
Fair Value - 12 Months or Greater	0		2,005,000
Fair Value - Total	2,579,000		4,587,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(11,000)		(10,000)
Unrealized Loss - 12 Months or Greater	0		(3,000)
Unrealized Loss - Total	(11,000)		(13,000)
Unrestricted [Member] | Corporate Bond Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	2,126,820,000		1,717,976,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,948,000		2,176,000
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(18,159,000)		(7,672,000)
Available-for-sale Securities	2,110,609,000		1,712,480,000
Unrestricted [Member] | U.S. government and government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	156,281,000		186,699,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	30,000		34,000
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(1,611,000)		(1,241,000)
Available-for-sale Securities	154,700,000		185,492,000
Unrestricted [Member] | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	2,591,000		4,600,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0		0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(11,000)		(13,000)
Available-for-sale Securities	2,580,000		4,587,000
Unrestricted [Member] | Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	70,385,000		106,973,000
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	70,385,000		106,973,000
Unrestricted [Member] | Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	20,097,000		11,024,000
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	$ 20,097,000		$ 11,024,000