Subsequent event (Details) (USD $) In Millions, except Share data, unless otherwise specified			0 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Jan. 15, 2013 Subsequent event item
Subsequent event
Number of investment banks with whom the entity entered into underwriting agreement			2
Number of shares of new stock issued during the period (in shares)			4,400,000
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001	$ 0.001
Sale of stock, price per share (in dollars per share)			$ 80.09
Proceeds from issuance of common stock			$ 352.4