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Noncontrolling Interests - Noncontrolling Interest Summary (Parenthetical) (Detail) - shares
shares in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Class A Common [Member]    
Noncontrolling Interest [Line Items]    
Outstanding limited partnership units, exchanged 8.4 8.4
Units outstanding 8.8 8.8
Common Units    
Noncontrolling Interest [Line Items]    
Outstanding limited partnership units, exchanged 6.8 7.2
Other Consolidated Entities [Member]    
Noncontrolling Interest [Line Items]    
Outstanding limited partnership units, exchanged 0.4 0.7
Other Consolidated Entities [Member] | Common Stock [Member]    
Noncontrolling Interest [Line Items]    
Outstanding limited partnership units 0.4 0.7