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Noncontrolling Interests - Noncontrolling Interest Summary (Parenthetical) (Detail) - shares
shares in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Noncontrolling Interest [Line Items]    
Business acquisition, common units issued 3.9 4.1
Class A Common [Member]    
Noncontrolling Interest [Line Items]    
Outstanding limited partnership units, exchanged 8.5 8.5
Units outstanding 8.9 8.9
Other Consolidated Entities [Member]    
Noncontrolling Interest [Line Items]    
Outstanding limited partnership units, exchanged 1.0 0.0
Other Consolidated Entities [Member] | Common Stock [Member]    
Noncontrolling Interest [Line Items]    
Outstanding limited partnership units 1.0 1.0