Marketable Securities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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	1,585,849		1,585,849		$ 1,380,487
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	11,884		11,884		6,241
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(9,519)		(9,519)		(18,987)
Available-for-sale Securities	1,588,214		1,588,214		1,367,741
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year	523,537		523,537		503,481
Maturities after one year through five years	1,004,308		1,004,308		815,041
Total	1,527,845		1,527,845		1,318,522
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	817,983		817,983		883,950
Fair Value - 12 Months or Greater	7,444		7,444		36,407
Fair Value - Total	825,427		825,427		920,357
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(9,514)		(9,514)		(18,929)
Unrealized Loss - 12 Months or Greater	(5)		(5)		(58)
Unrealized Loss - Total	(9,519)		(9,519)		(18,987)
Corporate Bond Securities [Member]
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	644,801		644,801		759,222
Fair Value - 12 Months or Greater	7,444		7,444		36,407
Fair Value - Total	652,245		652,245		795,629
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(2,656)		(2,656)		(4,685)
Unrealized Loss - 12 Months or Greater	(5)		(5)		(58)
Unrealized Loss - Total	(2,661)		(2,661)		(4,743)
U.S. government and government agency obligations
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	120,102		120,102		81,170
Fair Value - 12 Months or Greater	0		0		0
Fair Value - Total	120,102		120,102		81,170
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(570)		(570)		(641)
Unrealized Loss - 12 Months or Greater	0		0		0
Unrealized Loss - Total	(570)		(570)		(641)
Municipal bonds
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	4,113		4,113		7,141
Fair Value - 12 Months or Greater	0		0		0
Fair Value - Total	4,113		4,113		7,141
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(9)		(9)		(20)
Unrealized Loss - 12 Months or Greater	0		0		0
Unrealized Loss - Total	(9)		(9)		(20)
Equity Securities
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	48,967		48,967		36,417
Fair Value - 12 Months or Greater	0		0		0
Fair Value - Total	48,967		48,967		36,417
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(6,279)		(6,279)		(13,583)
Unrealized Loss - 12 Months or Greater	0		0		0
Unrealized Loss - Total	(6,279)		(6,279)		(13,583)
Unrestricted [Member] | Corporate Bond Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	1,350,232		1,350,232		1,076,964
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2,416		2,416		630
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(2,661)		(2,661)		(4,743)
Available-for-sale Securities	1,349,987		1,349,987		1,072,851
Unrestricted [Member] | U.S. government and government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	147,563		147,563		132,923
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	70		70		58
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(570)		(570)		(641)
Available-for-sale Securities	147,063		147,063		132,340
Unrestricted [Member] | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	4,624		4,624		7,663
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1		1		1
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(9)		(9)		(20)
Available-for-sale Securities	4,616		4,616		7,644
Unrestricted [Member] | Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	13,614		13,614		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	13,614		13,614		63,075
Unrestricted [Member] | Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	12,565		12,565		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	12,565		12,565		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	9,397		9,397		5,551
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(6,279)		(6,279)		(13,583)
Available-for-sale Securities	$ 60,369		$ 60,369		$ 49,219