Consolidated Statements of Stockholders' Equity (Parenthetical)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement Of Stockholders Equity [Abstract]
Options exercised for the purchase of class A common stock	232,448	96,267	54,921
Issuance of class A common stock under employee stock purchase plan	160,115		583,034
Issuance of restricted class A common stock	1,063,993	639,232	301,053
Conversion of Class B Common Stock to Class A Common Stock, shares	196,524	250,000	100,000