Income Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Calculation of basic and diluted net income per share
Allocation of undistributed earnings	$ 3,734	$ 2,811	$ 26,350	$ 7,800
Weighted-average common shares outstanding	111,214	104,185	110,334	103,198
Basic (in dollars per share)	$ 0.03	$ 0.03	$ 0.24	$ 0.08
Weighted-average effect of dilutive securities added
Number of shares used in diluted calculation	116,627	112,317	116,017	111,813
Diluted (in dollars per share)	$ 0.03	$ 0.03	$ 0.23	$ 0.07
Class A Common Stock
Calculation of basic and diluted net income per share
Allocation of undistributed earnings	3,108	1,863	21,810	4,386
Weighted-average common shares outstanding	92,560	69,063	91,325	58,024
Basic (in dollars per share)	$ 0.03	$ 0.03	$ 0.24	$ 0.08
Reallocation of undistributed earnings as a result of conversion of Class B to Class A shares	626	948	4,540	3,414
Undistributed Earnings Allocated to Participating Securities	3,734	2,811	26,350	7,800
Weighted-average effect of dilutive securities added
Conversion of Class B to Class A common shares outstanding	18,654	35,122	19,009	45,174
Number of shares used in diluted calculation	116,627	112,317	116,017	111,813
Diluted (in dollars per share)	$ 0.03	$ 0.03	$ 0.23	$ 0.07
Class B Common Stock
Calculation of basic and diluted net income per share
Allocation of undistributed earnings	626	948	4,540	3,414
Weighted-average common shares outstanding	18,654	35,122	19,009	45,174
Basic (in dollars per share)	$ 0.03	$ 0.03	$ 0.24	$ 0.08
Reallocation of undistributed earnings to Class B shares	103	122	791	4
Undistributed Earnings Allocated to Participating Securities	$ 729	$ 1,070	$ 5,331	$ 3,418
Weighted-average effect of dilutive securities added
Number of shares used in diluted calculation	22,763	42,760	23,473	48,993
Diluted (in dollars per share)	$ 0.03	$ 0.03	$ 0.23	$ 0.07
Employee stock options | Class A Common Stock
Weighted-average effect of dilutive securities added
Shares under share based payment award	4,294	7,794	4,645	8,362
Employee stock options | Class B Common Stock
Weighted-average effect of dilutive securities added
Shares under share based payment award	4,109	7,638	4,464	3,819
RSUs and other dilutive securities | Class A Common Stock
Weighted-average effect of dilutive securities added
Shares under share based payment award	1,119	338	1,038	253