Subsequent Events (Details) - Subsequent event	Feb. 08, 2018 $ / shares
Class A
Subsequent events
Quarterly dividend payable (in dollars per share)	$ 0.19
Class B
Subsequent events
Quarterly dividend payable (in dollars per share)	$ 0.19

The per share amount of a dividend declared, but not paid, as of the financial reporting date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586

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