Stockholders' Equity - Common Stock (Details) - $ / shares	Jun. 30, 2018	Dec. 31, 2017
Equity [Abstract]
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	9,000,000,000	9,000,000,000
Common stock, shares issued (in shares)	4,485,774,000	4,530,928,000
Common stock, shares outstanding (in shares)	4,485,774,000	4,527,742,000
Common stock reserved for issuance (in shares)	281,863,000