Marketable Securities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Marketable Securities, Realized Gain (Loss)	$ 0	$ 0	$ 0	$ 0
Amortized Cost Basis	1,883,892		1,883,892		$ 1,380,487
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	33,634		33,634		6,241
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(3,344)		(3,344)		(18,987)
Available-for-sale Securities	1,914,182		1,914,182		1,367,741
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year	586,879		586,879		503,481
Maturities after one year through five years	1,239,263		1,239,263		815,041
Total	1,826,142		1,826,142		1,318,522
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	820,872		820,872		883,950
Fair Value - 12 Months or Greater	117,488		117,488		36,407
Fair Value - Total	938,360		938,360		920,357
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(2,710)		(2,710)		(18,929)
Unrealized Loss - 12 Months or Greater	(634)		(634)		(58)
Unrealized Loss - Total	(3,344)		(3,344)		(18,987)
Corporate Bond Securities [Member]
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	675,374		675,374		759,222
Fair Value - 12 Months or Greater	110,545		110,545		36,407
Fair Value - Total	785,919		785,919		795,629
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(2,111)		(2,111)		(4,685)
Unrealized Loss - 12 Months or Greater	(580)		(580)		(58)
Unrealized Loss - Total	(2,691)		(2,691)		(4,743)
U.S. government and government agency obligations
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	141,393		141,393		81,170
Fair Value - 12 Months or Greater	6,943		6,943		0
Fair Value - Total	148,336		148,336		81,170
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(591)		(591)		(641)
Unrealized Loss - 12 Months or Greater	(54)		(54)		0
Unrealized Loss - Total	(645)		(645)		(641)
Municipal bonds
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	4,105		4,105		7,141
Fair Value - 12 Months or Greater	0		0		0
Fair Value - Total	4,105		4,105		7,141
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(8)		(8)		(20)
Unrealized Loss - 12 Months or Greater	0		0		0
Unrealized Loss - Total	(8)		(8)		(20)
Equity Securities
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months					36,417
Fair Value - 12 Months or Greater					0
Fair Value - Total					36,417
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months					(13,583)
Unrealized Loss - 12 Months or Greater					0
Unrealized Loss - Total					(13,583)
Unrestricted [Member] | Corporate Bond Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	1,564,391		1,564,391		1,076,964
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2,789		2,789		630
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(2,691)		(2,691)		(4,743)
Available-for-sale Securities	1,564,489		1,564,489		1,072,851
Unrestricted [Member] | U.S. government and government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	189,647		189,647		132,923
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	56		56		58
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(645)		(645)		(641)
Available-for-sale Securities	189,058		189,058		132,340
Unrestricted [Member] | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	4,614		4,614		7,663
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0		0		1
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(8)		(8)		(20)
Available-for-sale Securities	4,606		4,606		7,644
Unrestricted [Member] | Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	61,464		61,464		63,074
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0		0		1
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0		0		0
Available-for-sale Securities	61,464		61,464		63,075
Unrestricted [Member] | Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	6,525		6,525		42,612
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0		0		0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0		0		0
Available-for-sale Securities	6,525		6,525		42,612
Unrestricted [Member] | Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	57,251		57,251		57,251
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	30,789		30,789		5,551
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0		0		(13,583)
Available-for-sale Securities	$ 88,040		$ 88,040		$ 49,219