Common stock (Recognized expenses nonemployee stock-based compensation) (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Nonemployees	Dec. 31, 2012 Nonemployees	Dec. 31, 2011 Nonemployees	Dec. 31, 2010 Business Acquisition, Acquiree [Domain]
Share-based Goods and Nonemployee Services Transaction [Line Items]
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable							$ 100,000
Stock-based compensation expense	$ 8,667,000	$ 4,232,000	$ 1,458,000	$ 0	$ 29,000	$ 907,000