Marketable Securities - Maturities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Amortized Cost
Maturing in one year or less	$ 177,542	$ 115,748
Maturing after one year through five years	191,200	274,632
Total amortized cost	368,742	390,380
Estimate Fair Value
Maturing in one year or less	177,269	115,608
Maturing after one year through five years	190,242	273,340
Total estimated fair value	$ 367,511	$ 388,948