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Noncontrolling Interests - Noncontrolling Interest Summary (Parenthetical) (Detail) - shares
shares in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Class A Common [Member]    
Noncontrolling Interest [Line Items]    
Outstanding limited partnership units, exchanged 8.4 8.5
Units outstanding 8.8 8.9
Common Unit [Member]    
Noncontrolling Interest [Line Items]    
Outstanding limited partnership units, exchanged 3.9 4.1
Other Consolidated Entities [Member]    
Noncontrolling Interest [Line Items]    
Outstanding limited partnership units, exchanged 0.9 1.0
Other Consolidated Entities [Member] | Common Stock [Member]    
Noncontrolling Interest [Line Items]    
Outstanding limited partnership units 0.9 1.0