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Stock Transactions and Unit Redemptions - Unit Redemption and Share Repurchase Activity (Parenthetical) (Detail) - USD ($) $ / shares in Units, $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017
Class of Stock [Line Items] |  |  |
Average price for redemption of limited partnership units | $ 14.34 | $ 10.85 |
Aggregate purchase price of Class A common stock |  | $ 6,120 |
Class A Common Stock [Member] |  |  |
Class of Stock [Line Items] |  |  |
Repurchase of Class A common stock, shares Treasury Stock Shares Acquired | 0 | 552,955 | 1,386,769
Aggregate purchase price of Class A common stock |  | $ 6,100 |
Average price of Class A common stock |  | $ 11.07 |
Limited Partnership [Member] |  |  |
Class of Stock [Line Items] |  |  |
Redeemed limited partnership units | 1,700,000 | 2,900,000 |
Aggregate redemption price of limited partnership units | $ 24,600 | $ 31,600 |
Other Ownership Interest [Member] |  |  |
Class of Stock [Line Items] |  |  |
Redeemed limited partnership units | 5,000 | 15,700 |
Aggregate redemption price of limited partnership units | $ 74 | $ 200 |
Average price for redemption of founding/working partner units | $ 13.67 | $ 10.73 |

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- Definition Total change in each class of partners' capital accounts during the year due to redemptions and adjustments to redemption value. All partners include general, limited and preferred partners. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 5 -Subparagraph (SAB Topic 4.F) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187171-122770 Reference 2: 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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