Consolidation - Revenue and Expenses of Consolidated Investment Product (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income:
Total revenues	$ 79,850	$ 82,324	$ 80,085	$ 80,295	$ 86,115	$ 92,375	$ 99,656	$ 103,831	$ 322,554	$ 381,977	$ 450,598
Expenses:
Net Income Attributable to Common Stockholders	$ 12,426	$ 15,625	$ 8,088	$ 12,363	$ 6,636	$ (649)	$ 9,777	$ 19,342	48,502	35,106	97,700
Consolidated Investment Product [Member]
Income:
Realized and unrealized loss, net									(1,070)	(3,505)	0
Interest Income									12,893	2,157	0
Total revenues									11,823
Expenses:
Other operating expenses									3,944	0	0
Interest expense									11,292	$ 484	$ 0
Total Expense									15,236
Net Income Attributable to Common Stockholders									$ (3,413)