Investments - Amortized cost and estimated fair value of investments in available-for-sale securities, by contractual maturity (Details) $ in Thousands	Jun. 30, 2015 USD ($)
Cost
Mature in more than three years	$ 35,205
Securities with no contractual maturities	119,865
Total	155,070
Estimated Fair Value
Mature in more than three years	34,298
Securities with no contractual maturities	129,495
Total	$ 163,793