Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	$ 2,083,463	$ 1,380,487
Unrealized Gains	8,804	6,241
Unrealized (Losses)	(8,926)	(18,987)
Total fair value of available-for-sale marketable securities	2,083,341	1,367,741
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year	593,783	503,481
Maturities after one year through five years	1,426,773	815,041
Total	2,020,556	1,318,522
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	1,044,084	883,950
Fair Value - 12 Months or Greater	326,676	36,407
Fair Value - Total	1,370,760	920,357
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(5,343)	(18,929)
Unrealized Loss- 12 Months or Greater	(3,583)	(58)
Unrealized Loss - Total	(8,926)	(18,987)
Realized Gain (Loss) on Marketable Securities, Cost Method Investments, and Other Investments	8,300
Corporate bonds
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	930,970	759,222
Fair Value - 12 Months or Greater	256,750	36,407
Fair Value - Total	1,187,720	795,629
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(4,924)	(4,685)
Unrealized Loss- 12 Months or Greater	(2,748)	(58)
Unrealized Loss - Total	(7,672)	(4,743)
Corporate bonds | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	1,717,976	1,076,964
Unrealized Gains	2,176	630
Unrealized (Losses)	(7,672)	(4,743)
Total fair value of available-for-sale marketable securities	1,712,480	1,072,851
U.S. government and government agency obligations
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	110,532	81,170
Fair Value - 12 Months or Greater	67,921	0
Fair Value - Total	178,453	81,170
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(409)	(641)
Unrealized Loss- 12 Months or Greater	(832)	0
Unrealized Loss - Total	(1,241)	(641)
U.S. government and government agency obligations | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	186,699	132,923
Unrealized Gains	34	58
Unrealized (Losses)	(1,241)	(641)
Total fair value of available-for-sale marketable securities	185,492	132,340
Municipal Bonds [Member]
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	2,582	7,141
Fair Value - 12 Months or Greater	2,005	0
Fair Value - Total	4,587	7,141
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(10)	(20)
Unrealized Loss- 12 Months or Greater	(3)	0
Unrealized Loss - Total	(13)	(20)
Municipal Bonds [Member] | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	4,600	7,663
Unrealized Gains	0	1
Unrealized (Losses)	(13)	(20)
Total fair value of available-for-sale marketable securities	4,587	7,644
Commercial Paper [Member] | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	106,973	63,074
Unrealized Gains	0	1
Unrealized (Losses)	0	0
Total fair value of available-for-sale marketable securities	106,973	63,075
Certificates of Deposit [Member] | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	11,024	42,612
Unrealized Gains	0	0
Unrealized (Losses)	0	0
Total fair value of available-for-sale marketable securities	11,024	42,612
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment Losses, Investments	0	9,800	$ 0
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)
Equity Securities [Member] | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	56,191	57,251
Unrealized Gains	6,594	5,551
Unrealized (Losses)	0	(13,583)
Total fair value of available-for-sale marketable securities	62,785	49,219
Significant Other Observable Inputs (Level 2)
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment Losses, Investments	$ 0	$ 0	$ 0