Consolidated Statements of Changes in Equity Parenthetical - $ / shares	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class B Common Stock
Class of Stock [Line Items]
Common stock, par value per share (in dollars per share)	$ 0.001	$ 0.001	$ 0.001

Face amount or stated value per share of common stock.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_CommonStockParOrStatedValuePerShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant