Investments (Schedule of Maturities of Available-for-sale Securities) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 279,083	$ 251,847
Fair market value	279,330	251,877
Federal agency notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost, due in one year or less	117,412
Fair market value, due in one year or less	117,465
Amortized cost, due after year one through two years	51,073
Fair market value, due after year one through two years	51,117
Amortized cost	168,485	143,132
Fair market value	168,582	143,139
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost, due in one year or less	77,447
Fair market value, due in one year or less	77,365
Amortized cost, due after year one through two years	32,683
Fair market value, due after year one through two years	32,618
Amortized cost	110,130	108,371
Fair market value	$ 109,983	$ 108,115