Cash and Investments - Contract Maturity Date (Details) $ in Thousands	Jun. 30, 2016 USD ($)
Available-for-sale investments by contractual maturity date
Due in one year or less, amortized cost	$ 1,215,357
Due in one year or less, estimated fair market value	1,215,893
Due after one year through two years, amortized cost	1,009,638
Due after one year through two years, estimated fair market value	1,012,282
Due after two years through three years, amortized cost	360,046
Due after two years through three years, estimated fair market value	363,534
Total, amortized cost	2,585,041
Total, estimated fair market value	$ 2,591,709