Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	$ 1,380,487	$ 856,429
Unrealized Gains	6,241	14,623
Unrealized (Losses)	(18,987)	(2,769)
Total fair value of available-for-sale marketable securities	1,367,741	868,283
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year	503,482	236,121
Maturities after one year through five years	815,040	600,695
Total	1,318,522	836,816
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	883,950	731,331
Fair Value - 12 Months or Greater	36,407	23,749
Fair Value - Total	920,357	755,080
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(18,929)	(2,677)
Unrealized Loss- 12 Months or Greater	(58)	(92)
Unrealized Loss - Total	(18,987)	(2,769)
Available-for-sale Securities, Gross Realized Losses			$ 0
Corporate bonds
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	759,222	668,199
Fair Value - 12 Months or Greater	36,407	23,749
Fair Value - Total	795,629	691,948
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(4,685)	(2,473)
Unrealized Loss- 12 Months or Greater	(58)	(92)
Unrealized Loss - Total	(4,743)	(2,565)
Corporate bonds | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	1,076,964	770,092
Unrealized Gains	630	156
Unrealized (Losses)	(4,743)	(2,565)
Total fair value of available-for-sale marketable securities	1,072,851	767,683
U.S. government and government agency obligations
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	81,170	51,215
Fair Value - 12 Months or Greater	0	0
Fair Value - Total	81,170	51,215
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(641)	(193)
Unrealized Loss- 12 Months or Greater	0	0
Unrealized Loss - Total	(641)	(193)
U.S. government and government agency obligations | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	132,923	51,402
Unrealized Gains	58	0
Unrealized (Losses)	(641)	(193)
Total fair value of available-for-sale marketable securities	132,340	51,209
Municipal Bonds [Member]
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	7,141	11,917
Fair Value - 12 Months or Greater	0	0
Fair Value - Total	7,141	11,917
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(20)	(11)
Unrealized Loss- 12 Months or Greater	0	0
Unrealized Loss - Total	(20)	(11)
Municipal Bonds [Member] | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	7,663	17,930
Unrealized Gains	1	5
Unrealized (Losses)	(20)	(11)
Total fair value of available-for-sale marketable securities	7,644	17,924
Commercial Paper [Member] | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	63,074
Unrealized Gains	1
Unrealized (Losses)	0
Total fair value of available-for-sale marketable securities	63,075
Certificates of Deposit [Member] | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	42,612
Unrealized Gains	0
Unrealized (Losses)	0
Total fair value of available-for-sale marketable securities	42,612
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment Losses, Investments	9,800	0	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	57,251	17,005
Unrealized Gains	5,551	14,462
Unrealized (Losses)	(13,583)	0
Total fair value of available-for-sale marketable securities	49,219	31,467
Significant Other Observable Inputs (Level 2)
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment Losses, Investments	$ 0	$ 0	$ 0