Stock incentive plans (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Recognized stock-based compensation expense
Total stock-based compensation	$ 2,735,000	$ 3,423,000	$ 867,000
Number of Options
Outstanding at the beginning of the period (in shares)	6,601,000	5,288,000	5,281,000
Granted (in shares)	1,391,000	2,381,000	53,000
Exercised (in shares)	(1,899,000)	(1,015,000)
Cancelled/forfeited (in shares)	(1,048,000)	(53,000)	(44,000)
Outstanding at the end of the period (in shares)	5,045,000	6,601,000	5,288,000	5,281,000
Vested and expected to vest at the end of the period (in shares)	4,855,000
Exercisable at the end of the period (in shares)	2,776,000
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 5.50	$ 1.25	$ 1.25
Granted (in dollars per share)	$ 8.29	$ 13.02	$ 2.85
Exercised (in dollars per share)	$ 1.36	$ 1.01	$ 0.90
Cancelled/forfeited (in dollars per share)	$ 11.85	$ 6.58	$ 1.45
Outstanding at the end of the period (in dollars per share)	$ 6.50	$ 5.50	$ 1.25	$ 1.25
Vested and expected to vest at the end of the period (in dollars per share)	$ 6.37
Exercisable at the end of the period (in dollars per share)	$ 3.46
Weighted-Average Remaining Contract Life (years)
Outstanding	6 years 5 months 8 days	7 years	6 years 3 months 18 days	7 years 3 months 18 days
Vested and expected to vest at the end of the period	6 years 4 months 2 days
Exercisable at the end of the period	4 years 6 months 7 days
Aggregate Intrinsic Value
Outstanding at the beginning of the period	30,996,000	38,279,000	8,471,000
Outstanding at the end of the period	14,742,000	30,996,000	38,279,000	8,471,000
Vested and expected to vest at the end of the period	14,690,000
Exercisable at the end of the period	13,964,000
Total intrinsic value of stock options exercised (in dollars)	14,901,000	9,754,000	3,000
Stock-based compensation expense for unvested awards not recognized (in dollars)	9,130,000
Weighted average period over which unrecognized compensation expense is expected to be recognized	3 years 2 months 12 days
Cash proceeds from exercise of stock option	2,573,000	1,028,000	2,000
Weighted average grant-date fair value of stock options granted (in dollars per share)	$ 8.10	$ 6.68	$ 1.80
Tax benefits for the deductions from stock option exercises	372,000	246,000	0
Network operations
Recognized stock-based compensation expense
Total stock-based compensation	352,000	463,000	131,000
Development and technology
Recognized stock-based compensation expense
Total stock-based compensation	352,000	577,000	115,000
Selling and marketing
Recognized stock-based compensation expense
Total stock-based compensation	571,000	650,000	171,000
General and administrative
Recognized stock-based compensation expense
Total stock-based compensation	$ 1,460,000	$ 1,733,000	$ 450,000