Marketable Securities Schedule of Fair Value and Unrealized Losses of Debt Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	$ 1,636,873	$ 1,044,084
Unrealized Loss - Less than 12 months	(15,716)	(5,343)
Fair Value - 12 Months or Greater	289,973	326,676
Unrealized Loss - 12 Months or Greater	(4,868)	(3,583)
Fair Value - Total	1,926,846	1,370,760
Unrealized Loss - Total	(20,584)	(8,926)
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	1,588,876	930,970
Unrealized Loss - Less than 12 months	(15,052)	(4,924)
Fair Value - 12 Months or Greater	218,109	256,750
Unrealized Loss - 12 Months or Greater	(3,935)	(2,748)
Fair Value - Total	1,806,985	1,187,720
Unrealized Loss - Total	(18,987)	(7,672)
U.S. government and government agency obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	45,936	110,532
Unrealized Loss - Less than 12 months	(656)	(409)
Fair Value - 12 Months or Greater	71,864	67,921
Unrealized Loss - 12 Months or Greater	(933)	(832)
Fair Value - Total	117,800	178,453
Unrealized Loss - Total	(1,589)	(1,241)
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value - Less than 12 Months	2,061	2,582
Unrealized Loss - Less than 12 months	(8)	(10)
Fair Value - 12 Months or Greater	0	2,005
Unrealized Loss - 12 Months or Greater	0	(3)
Fair Value - Total	2,061	4,587
Unrealized Loss - Total	$ (8)	$ (13)