Fair value of financial instruments (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Fair value of financial instruments
Investments lower than the amortized cost basis or any investments in an unrealized loss position for a significant period	$ 0	$ 0
Maximum period of investments for unrealized loss position	12 months
Transfers of assets between the fair value measurement classifications	0
Fair value measurements on a recurring basis | Total
Assets:
Total	59,703,000	31,391,000
Fair value measurements on a recurring basis | Quoted Prices in Active Markets (Level 1)
Assets:
Total	43,188,000	14,776,000
Fair value measurements on a recurring basis | Significant Other Observable Inputs (Level 2)
Assets:
Total	16,515,000	16,615,000
Fair value measurements on a recurring basis | Cash and cash equivalents | Total
Assets:
Total	43,188,000	14,776,000
Fair value measurements on a recurring basis | Cash and cash equivalents | Quoted Prices in Active Markets (Level 1)
Assets:
Total	43,188,000	14,776,000
Fair value measurements on a recurring basis | Marketable securities: Corporate debt securities | Total
Assets:
Total	16,515,000	16,615,000
Fair value measurements on a recurring basis | Marketable securities: Corporate debt securities | Significant Other Observable Inputs (Level 2)
Assets:
Total	$ 16,515,000	$ 16,615,000