Investments (Schedule of Maturities 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
Fair value	326,968	251,877
Federal agency notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost, due in one year or less	237,658
Fair value, due in one year or less	237,764
Amortized cost, due after year one through two years	48,562
Fair value, due after year one through two years	48,528
Amortized cost	286,220	143,132
Fair value	286,292	143,139
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost, due in one year or less	26,612
Fair value, due in one year or less	26,567
Amortized cost, due after year one through two years	13,485
Fair value, due after year one through two years	13,485
Amortized cost	40,097	108,371
Fair value	$ 40,052	$ 108,115