Loss recognized during the period that results from the write-down of goodwill after comparing the implied fair value of reporting unit goodwill with the carrying amount of that goodwill. Goodwill is assessed at least annually for impairment.

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+ Details

Summary of significant accounting policies (Details 3) (USD $)	12 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011
Goodwill
Number of reporting units	1
Number of operating segments	1
Number of reportable segments	1
Goodwill impairment charges	$ 0	$ 0
Computer equipment \| Minimum
Property and equipment
Estimated useful lives	2 years
Computer equipment \| Maximum
Property and equipment
Estimated useful lives	5 years
Software \| Minimum
Property and equipment
Estimated useful lives	2 years
Software \| Maximum
Property and equipment
Estimated useful lives	5 years
Office equipment \| Minimum
Property and equipment
Estimated useful lives	3 years
Office equipment \| Maximum
Property and equipment
Estimated useful lives	5 years
Leasehold improvements \| Minimum
Property and equipment
Estimated useful lives	3 years
Leasehold improvements \| Maximum
Property and equipment
Estimated useful lives	15 years