Investments - Schedule of Fair Value of Held-to-maturity Securities Recorded at Amortized Cost (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Held To Maturity Securities [Line Items]
Fair market value	$ 682,098	$ 1,223,755
Corporate securities
Schedule Of Held To Maturity Securities [Line Items]
Fair market value	227,854	492,955
U.S. Treasury securities
Schedule Of Held To Maturity Securities [Line Items]
Fair market value	161,778	105,064
Commercial paper
Schedule Of Held To Maturity Securities [Line Items]
Fair market value	$ 292,466	604,986
Certificates of deposit
Schedule Of Held To Maturity Securities [Line Items]
Fair market value		$ 20,750