Schedule I - Condensed Statements of Financial Condition (Parent Company Only) (Parenthetical) (Detail) - $ / shares	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	15,000,000	15,000,000
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	500,000,000	500,000,000
Common stock, shares issued	129,766,091	129,766,091
Common stock held by subsidiaries, shares	10,747,142	7,628,786
Series A Preferred Stock [Member]
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0	7,921
Series B Preferred Stock [Member]
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0	0
Parent Company [Member]
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	15,000,000	15,000,000
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	500,000,000	500,000,000
Common stock, shares issued	129,766,091	129,766,091
Common stock held by subsidiaries, shares	10,747,142	7,628,786
Parent Company [Member] | Series A Preferred Stock [Member]
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Parent Company [Member] | Series B Preferred Stock [Member]
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0