Aggregate dividends declared during the period for each share of common stock outstanding.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

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- Definition

Date the declared dividend will be paid, in CCYY-MM-DD format.

+ References

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=98513485&loc=d3e4304-108586

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Date the holder must own the stock to be entitled to the dividend, in CCYY-MM-DD format.

+ References

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Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

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Subsequent Events - Additional Information (Detail) - $ / shares	1 Months Ended		3 Months Ended		6 Months Ended
	Feb. 28, 2018	Aug. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Subsequent Event [Line Items]
Cash dividends declared per share of common stock			$ 0.00	$ 0.00	$ 0.10	$ 0.10
Cash dividends per common share, date of dividend payable		Oct. 18, 2017
Cash dividends per common share, date of record for dividend		Sep. 13, 2017
Class A Common Stock [Member]
Subsequent Event [Line Items]
Cash dividends declared per share of common stock		$ 0.10
Class B Common Stock [Member]
Subsequent Event [Line Items]
Cash dividends declared per share of common stock		$ 0.10
Subsequent Event [Member]
Subsequent Event [Line Items]
Cash dividends per common share, date of dividend payable	Apr. 18, 2018
Cash dividends per common share, date of record for dividend	Mar. 14, 2018
Subsequent Event [Member] \| Class A Common Stock [Member]
Subsequent Event [Line Items]
Cash dividends declared per share of common stock	$ 0.10
Subsequent Event [Member] \| Class B Common Stock [Member]
Subsequent Event [Line Items]
Cash dividends declared per share of common stock	$ 0.10