Unconsolidated Entities - Summary of Operating Information and Financial Position of Unconsolidated Co-investment Ventures (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity Method Investments And Joint Ventures [Abstract]
Deferred gain recognize	$ 667.3	$ 469.9	$ 430.7
Additional costs associated with investment in a venture	94.2	124.1	122.1
Advances to venture	$ 210.0	$ 166.1	$ 189.7