Subsequent Events - Additional Information (Detail)				3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 31, 2017 shares	Jul. 25, 2017 $ / shares	Jul. 18, 2017 USD ($) Director	Jun. 30, 2017 shares	Jun. 30, 2016 shares	Jun. 30, 2017 shares	Jun. 30, 2016 shares	Dec. 31, 2016 shares	May 15, 2017
Class A Common Stock [Member]
Subsequent Event [Line Items]
Common stock, issued during the period | shares						33,616		2,004,533
Class B Common Stock [Member]
Subsequent Event [Line Items]
Common stock, issued during the period | shares				0	0	0	0
Subsequent Event
Subsequent Event [Line Items]
Contribution to investments			$ 100,000,000
Percentage of capital contributed			27.00%
Percentage of cumulative annual preferred return			5.00%
Subsequent Event | Class A Common Stock [Member]
Subsequent Event [Line Items]
Dividend declared per share | $ / shares		$ 0.18
Dividend payable declared date		Jul. 25, 2017
Dividend payable, date to be paid		Aug. 28, 2017
Dividend payable record date		Aug. 14, 2017
Subsequent Event | Class A Common Stock [Member] | Controlled Equity Offerings [Member]
Subsequent Event [Line Items]
Common stock, issued during the period | shares	500,000
Subsequent Event | Class B Common Stock [Member]
Subsequent Event [Line Items]
Dividend declared per share | $ / shares		$ 0.18
Dividend payable declared date		Jul. 25, 2017
Dividend payable, date to be paid		Aug. 28, 2017
Dividend payable record date		Aug. 14, 2017
Subsequent Event | Cantor [Member]
Subsequent Event [Line Items]
Contribution to investments			$ 267,000,000
Percentage of capital contributed			73.00%
Subsequent Event | Cantor [Member] | Maximum [Member]
Subsequent Event [Line Items]
Initial net losses of partnership			$ 37,000,000
Barkeley Point | Subsequent Event
Subsequent Event [Line Items]
Total consideration agreed to acquire			$ 875,000,000
Number of directors in special committee | Director			4
Spring 11 [Member] | Two Subsidiaries of Cantor [Member]
Subsequent Event [Line Items]
Ownership percentage acquired									75.00%