Earnings Per Share - Additional Information (Detail)	9 Months Ended
Sep. 30, 2017
Class A Common Stock [Member]
Earnings Per Share [Line Items]
Common stock, voting rights	One vote per share
Class B Common Stock [Member]
Earnings Per Share [Line Items]
Common stock, voting rights	Ten votes per share

Description of voting rights of common stock. Includes eligibility to vote and votes per share owned. Include also, if any, unusual voting rights.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21475-112644

Name:	us-gaap_CommonStockVotingRights
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration