Short-Term and Long-Term Investments (Schedule of Investments That Have Been in a Continuous Unrealized Loss Position) (Details) $ in Thousands	Mar. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Continuous Loss Position, Unrealized Loss, Less Than 12 Months	$ 368,454
Gross Unrealized Losses, Continuous Loss Position, Unrealized Loss, Less Than 12 Months	(778)
Fair Value, Continuous Loss Position, Unrealized Loss, 12 Months or Greater	77,680
Gross Unrealized Losses, Continuous Loss Position, 12 Months or Greater	(70)
Fair Value, Continuous Loss Position, Unrealized Loss, Total	446,134
Gross Unrealized Losses, Continuous Loss Position, Total	(848)
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Continuous Loss Position, Unrealized Loss, Less Than 12 Months	164,402
Gross Unrealized Losses, Continuous Loss Position, Unrealized Loss, Less Than 12 Months	(383)
Fair Value, Continuous Loss Position, Unrealized Loss, 12 Months or Greater	60,125
Gross Unrealized Losses, Continuous Loss Position, 12 Months or Greater	(62)
Fair Value, Continuous Loss Position, Unrealized Loss, Total	224,527
Gross Unrealized Losses, Continuous Loss Position, Total	(445)
Municipal Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Continuous Loss Position, Unrealized Loss, Less Than 12 Months	45,514
Gross Unrealized Losses, Continuous Loss Position, Unrealized Loss, Less Than 12 Months	(49)
Fair Value, Continuous Loss Position, Unrealized Loss, 12 Months or Greater	12,557
Gross Unrealized Losses, Continuous Loss Position, 12 Months or Greater	(7)
Fair Value, Continuous Loss Position, Unrealized Loss, Total	58,071
Gross Unrealized Losses, Continuous Loss Position, Total	(56)
U.S. Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Continuous Loss Position, Unrealized Loss, Less Than 12 Months	93,900
Gross Unrealized Losses, Continuous Loss Position, Unrealized Loss, Less Than 12 Months	(120)
Fair Value, Continuous Loss Position, Unrealized Loss, 12 Months or Greater	4,998
Gross Unrealized Losses, Continuous Loss Position, 12 Months or Greater	(1)
Fair Value, Continuous Loss Position, Unrealized Loss, Total	98,898
Gross Unrealized Losses, Continuous Loss Position, Total	(121)
U.S. Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Continuous Loss Position, Unrealized Loss, Less Than 12 Months	64,638
Gross Unrealized Losses, Continuous Loss Position, Unrealized Loss, Less Than 12 Months	(226)
Fair Value, Continuous Loss Position, Unrealized Loss, 12 Months or Greater	0
Gross Unrealized Losses, Continuous Loss Position, 12 Months or Greater	0
Fair Value, Continuous Loss Position, Unrealized Loss, Total	64,638
Gross Unrealized Losses, Continuous Loss Position, Total	$ (226)