Common stock (Nonemployee fair value stock options assumptions) (Details)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Share-based Goods and Nonemployee Services Transaction [Line Items]
Risk-free interest rate	1.77%
Expected life	8 years 8 months 1 day
Expected volatility	46.70%
Dividend yield	0.00%	0.00%
Minimum
Share-based Goods and Nonemployee Services Transaction [Line Items]
Risk-free interest rate		1.63%
Expected life		8 years 9 months 29 days
Expected volatility		45.00%
Maximum
Share-based Goods and Nonemployee Services Transaction [Line Items]
Risk-free interest rate		3.41%
Expected life		10 years
Expected volatility		54.00%