Fair value measurement - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Financial instruments measured at fair value on a recurring basis, transfers out of Level 3	$ 0	$ 0	$ (7,616,000)	$ 0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0	2,405,000	0
Cost To Sell	6,000,000	2,000,000
Fair Value Level 2 To Level 1 Transfers Amount	0	0
Fair Value Level 1 To Level 2 Transfers Amount	$ 0	$ 0