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	Jun. 30, 2013
Amortized Cost
Schedule of Available-for-sale Securities [Line Items]
Due within one year	$ 57,758
Due within two years	85,315
Due after two years	22,666
Total	165,739
Estimated Fair Value
Schedule of Available-for-sale Securities [Line Items]
Due within one year	57,850
Due within two years	85,476
Due after two years	22,572
Total	$ 165,898