Investments - Amortized cost and estimated fair value of investments in available-for-sale securities, by contractual maturity (Details) $ in Thousands	Sep. 30, 2015 USD ($)
Cost
Mature in more than three years	$ 34,034
Securities with no contractual maturities	139,077
Total	173,111
Estimated Fair Value
Mature in more than three years	29,844
Securities with no contractual maturities	141,989
Total	$ 171,833