Investments (Schedule of Maturities of Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 281,336	$ 317,488
Fair value	281,569	317,172
Federal agency notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost, due in one year or less	172,432
Fair value, due in one year or less	172,433
Amortized cost, due after year one through two years	75,768
Fair value, due after year one through two years	75,731
Amortized cost	248,200	285,276
Fair value	248,164	284,757
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost, due in one year or less	18,990
Fair value, due in one year or less	18,990
Amortized cost, due after year one through two years	13,490
Fair value, due after year one through two years	13,504
Amortized cost	32,480	31,730
Fair value	$ 32,494	$ 31,672