Cash and Investments - Contract Maturity Date (Details) $ in Thousands	Sep. 30, 2016 USD ($)
Available-for-sale investments by contractual maturity date
Due in one year or less, amortized cost	$ 1,517,136
Due in one year or less, estimated fair market value	1,517,691
Due after one year through two years, amortized cost	789,018
Due after one year through two years, estimated fair market value	790,476
Due after two years through three years, amortized cost	358,236
Due after two years through three years, estimated fair market value	359,716
Total, amortized cost	2,664,390
Total, estimated fair market value	$ 2,667,883