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

Average redemption price per unit for founding/working partner units.

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- Definition

Limited partnership units average redemption price per unit.

+ References

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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

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-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

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The number of units redeemed during the year of each class of partners' capital account. Units represent shares of ownership of the general, limited, and preferred partners.

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The cash outflow to reacquire common stock during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (a)
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Total cost of shares repurchased divided by the total number of shares repurchased.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 45
-Paragraph 1
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Number of shares that have been repurchased during the period and are being held in treasury.

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Reference 1: 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

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-Section S99
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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