Shareholders' Equity - Classes of Common Stock (Details)	12 Months Ended
Dec. 31, 2017 vote / shares
Common Class B [Member]
Class of Stock [Line Items]
Votes per common share	10
Common Class A [Member]
Class of Stock [Line Items]
Votes per common share	1

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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