Fair value measurements (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Unobservable Input Reconciliation [Roll Forward]
Balance beginning of period	$ 4,753,000	$ 14,532,000
Expense (income) included in earnings	(3,005,000)	221,000
Settlements	(1,748,000)	(10,000,000)
Purchases, sales and issuances	0	0
Transfers in/(out) of Level 3	0	0
Balance End of period	0	4,753,000
Long-term indebtedness [Abstract]
Total long-term indebtedness	62,774,000	59,454,000
Fair value of long-term indebtedness	62,800,000	59,500,000
Fair Value, Measurements, Recurring [Member]
Assets:
Investment in money market funds	42,720,000	73,005,000
U.S. Treasury securities		1,966,000
Total assets	42,720,000	74,971,000
Liabilities:
Contingent value rights		4,753,000
Total liabilities		4,753,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Investment in money market funds	42,720,000	73,005,000
U.S. Treasury securities		0
Total assets	42,720,000	73,005,000
Liabilities:
Contingent value rights		0
Total liabilities		0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets:
Investment in money market funds	0	0
U.S. Treasury securities		1,966,000
Total assets	0	1,966,000
Liabilities:
Contingent value rights		0
Total liabilities		0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets:
Investment in money market funds	0	0
U.S. Treasury securities		0
Total assets	0	0
Liabilities:
Contingent value rights		4,753,000
Total liabilities		$ 4,753,000