Cash, cash equivalents and short-term investments - Summary of cost and estimated fair value of short-term investmetns (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013
Amortized Cost
Schedule of Available-for-sale Securities [Line Items]
Due within one year	$ 72,091
Due within two years	70,333
Due after two years	22,884
Total	165,308
Estimated Fair Value
Schedule of Available-for-sale Securities [Line Items]
Due within one year	72,204
Due within two years	70,542
Due after two years	22,906
Total	$ 165,652