Investments (Schedule of Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 326,736	$ 251,847
Gross unrealized gains	311	320
Gross unrealized losses	(79)	(290)
Fair value	326,968	251,877
Federal agency notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	286,220	143,132
Gross unrealized gains	106	40
Gross unrealized losses	(34)	(33)
Fair value	286,292	143,139
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	40,097	108,371
Gross unrealized gains	0	1
Gross unrealized losses	(45)	(257)
Fair value	40,052	108,115
Mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	419	344
Gross unrealized gains	205	279
Gross unrealized losses	0	0
Fair value	$ 624	$ 623