Equity - Additional Information (Detail) - USD ($)	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	Feb. 02, 2018	May 31, 2013
Class of Stock [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
Subsequent Event [Member]
Class of Stock [Line Items]
Remaining authorized amount under stock repurchase program							$ 429,000,000
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
Class A Common Stock [Member]
Class of Stock [Line Items]
Aggregate amount of shares authorized to be repurchased								$ 500,000,000
Number of shares repurchased					0
Cash dividends declared per share of common stock		$ 0.10
Class A Common Stock [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Number of shares repurchased							5,200,000
Value of shares purchased in the period							$ 71,000,000
Cash dividends declared per share of common stock	$ 0.10
Class B Common Stock [Member]
Class of Stock [Line Items]
Cash dividends declared per share of common stock		$ 0.10
Class B Common Stock [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Cash dividends declared per share of common stock	$ 0.10