Business Segments - Segment Reporting, Reconciliation of Revenues, Operating Income and Assets (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Total revenues	$ 643,201	$ 580,622	$ 510,404	$ 462,847	$ 450,865	$ 415,151	$ 460,089	$ 434,682	$ 2,197,074	$ 1,760,787	$ 1,750,486
Europe Private Capital Segment [Member]
Segment Reporting Information [Line Items]
Goodwill	25,300				25,300				25,300	25,300	0
United States [Member]
Segment Reporting Information [Line Items]
Total revenues									1,900,000	1,400,000	1,100,000
Long-lived assets attributable to the United States	$ 23,200,000				$ 17,300,000				$ 23,200,000	$ 17,300,000	$ 0