Joint Ventures and Investments - Other Equity Investments (Parenthetical) (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		12 Months Ended
	Jul. 31, 2016	Mar. 31, 2018	Sep. 30, 2016	Dec. 31, 2017	Jan. 31, 2018	Jan. 01, 2018	Jul. 31, 2017
Schedule Of Investments [Line Items]
Fair value of available for sale securities		$ 1.0
ASU 2016-01
Schedule Of Investments [Line Items]
Cumulative loss in fair value of available-for-sale securities reclassified from accumulated other comprehensive income to accumulated losses resulted from new accounting standard adoption						$ 3.2
Complex Media
Schedule Of Investments [Line Items]
Cash received in connection with sale of interest	$ 35.3
Additional amount held in escrow	$ 3.7						$ 3.7
Period for amount held in escrow	12 months
Period following the closing date for escrow account	18 months
Gain on sale of investments		$ 1.0	$ 10.2	$ 2.7
Cash received from escrow					$ 1.0		$ 2.7