Marketable Securities - Schedule of Fair Value and Unrealized Losses of Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	$ 1,425,290	$ 1,044,084
Unrealized Loss - Less than 12 months	(11,148)	(5,343)
Fair Value - 12 Months or Greater	503,282	326,676
Unrealized Loss - 12 Months or Greater	(8,504)	(3,583)
Fair Value - Total	1,928,572	1,370,760
Unrealized Loss - Total	(19,652)	(8,926)
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	1,417,464	930,970
Unrealized Loss - Less than 12 months	(10,946)	(4,924)
Fair Value - 12 Months or Greater	406,594	256,750
Unrealized Loss - 12 Months or Greater	(7,178)	(2,748)
Fair Value - Total	1,824,058	1,187,720
Unrealized Loss - Total	(18,124)	(7,672)
U.S. government and government agency obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	7,826	110,532
Unrealized Loss - Less than 12 months	(202)	(409)
Fair Value - 12 Months or Greater	95,690	67,921
Unrealized Loss - 12 Months or Greater	(1,321)	(832)
Fair Value - Total	103,516	178,453
Unrealized Loss - Total	(1,523)	(1,241)
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	0	2,582
Unrealized Loss - Less than 12 months	0	(10)
Fair Value - 12 Months or Greater	998	2,005
Unrealized Loss - 12 Months or Greater	(5)	(3)
Fair Value - Total	998	4,587
Unrealized Loss - Total	$ (5)	$ (13)