Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Common stock, par value (in USD per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	300,000,000	300,000,000
Class A Common Stock
Common stock, shares issued (in shares)	129,558,000	114,390,000
Common stock, shares outstanding (in shares)	83,567,000	68,339,000
Common stock, shares held in treasury (in shares)	45,991,000	45,991,000