Common stock (Fair value option pricing model) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Options - intrinsic value and grant date fair value [Abstract]
Options vested and exercisable (Aggregate intrinsic value)	$ 21,169
Weighted-average grant-date fair value (in dollars per share)	$ 7.34	$ 10.19	$ 3.30
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life (years)			5 years 9 months 7 days
Dividend yield	0.00%	0.00%	0.00%
Stock options | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life (years)	5 years 2 months 23 days	5 years 5 months 27 days
Expected Volatility Rate	47.90%	44.70%	44.00%
Risk-free interest rate	0.72%	0.98%	1.90%
Stock options | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life (years)	5 years 7 months 6 days	5 years 8 months 23 days
Expected Volatility Rate	48.70%	47.60%	44.50%
Risk-free interest rate	1.03%	2.48%	2.63%