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Stock Transactions and Unit Redemptions - Changes in Shares of Class A Common Stock Outstanding (Detail) - Class A Common Stock [Member] - shares	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Class of Stock [Line Items]
Shares outstanding at beginning of period	256,968,372	244,869,624	244,869,624
Share issuances:
Issuance of Class A common stock for general corporate purposes	17,923,714
Exchanges of limited partnership interests	2,787,190	3,410,672	9,179,295
Compensation awards	979,344
Acquisition related issuances	317,096	373,078
Vesting of restricted stock units (RSUs)	280,884	356,389
Other issuances of Class A common stock	22,644	9,942
Treasury stock repurchases	0	(552,955)	(1,386,769)
Shares outstanding at end of period	279,279,244	248,466,750	256,968,372

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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