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Stock Transactions and Unit Redemptions - Changes in Shares of Class A Common Stock Outstanding (Detail) - shares
1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Aug. 31, 2018
Jul. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Share issuances:                  
Treasury stock repurchases (5,574) (696) (1,993)   (726,539)   (734,802)    
Forfeitures of restricted Class A common stock             (194,000) (107,000)  
Class A Common Stock [Member]                  
Class of Stock [Line Items]                  
Shares outstanding at beginning of period     290,909,843 290,909,843   251,057,086 256,968,372 244,869,624 244,869,624
Share issuances:                  
Issuance of Class A common stock for general corporate purposes             24,923,714    
Exchanges of limited partnership interests       2,474,658   1,434,050 9,949,679 6,795,187 9,179,295
Compensation awards             979,344    
Acquisition related issuances       78,678   82,907 1,083,150 1,424,690  
Vesting of restricted stock units (RSUs)       82,345   42,295 464,019 479,313  
Other issuances of Class A common stock       23,767   11,874 72,182 45,490  
Treasury stock repurchases       (8,263)   (295,871) (734,802) (1,246,365) (1,386,769)
Forfeitures of restricted Class A common stock       (49,189)   (71,251) (193,819) (106,849) (183,246)
Shares outstanding at end of period 293,511,839     293,511,839 290,909,843 252,261,090 293,511,839 252,261,090 256,968,372