Shareholders' Equity - Summary of Dividends and Distributions(Detail) - $ / shares									6 Months Ended	12 Months Ended
	Oct. 24, 2017	Jul. 25, 2017	Apr. 25, 2017	Feb. 28, 2017	Nov. 01, 2016	Aug. 02, 2016	May 03, 2016	Mar. 08, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Equity [Abstract]
Declaration Date	Oct. 24, 2017	Jul. 25, 2017	Apr. 25, 2017	Feb. 28, 2017	Nov. 01, 2016	Aug. 02, 2016	May 03, 2016	Mar. 08, 2016
Record Date	Dec. 29, 2017	Sep. 29, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016
Payment Date	Jan. 11, 2018	Oct. 12, 2017	Jul. 13, 2017	Apr. 13, 2017	Jan. 12, 2017	Oct. 13, 2016	Jul. 14, 2016	Apr. 14, 2016
Dividends per Class A and Class C Common Share	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.50	$ 1.00	$ 1.00