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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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- Definition Number of shares of stock issued during the period pursuant to acquisitions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(28)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
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