Short-Term Borrowings - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Mar. 23, 2015	Jan. 27, 2015	Aug. 30, 2012
Line Of Credit Facility [Line Items]
Fund-based and non-fund-based credit facilities limits available		$ 15,382,000	$ 15,781,000
Utilization of credit facility for non fund-based usage		10,980,000	10,301,000
Credit facility, amount utilized		160,978,000	22,947,000
Short-term borrowings		$ 160,000,000	$ 21,500,000
Revolving credit facility, expiration month and year		2020-06
Margin over LIBOR	1.50%	1.50%	1.50%
Percentage of commitment fee		0.25%	0.25%
Line of credit covenant condition		The credit agreement contains certain customary covenants, including a maximum leverage covenant and a minimum interest coverage ratio.
Morgan Stanley Senior Funding, Inc
Line Of Credit Facility [Line Items]
Short-term borrowings				$ 737,500,000	$ 672,500,000
Short term borrowings fixed interest rate		2.00%
Debt issuance cost		$ 1,045,000
Interest expense		235,000
Revolving Credit Facility
Line Of Credit Facility [Line Items]
Credit facility, maximum borrowing capacity		350,000,000				$ 250,000,000
Non-Fund-Based Credit Facility
Line Of Credit Facility [Line Items]
Credit facility, amount utilized		$ 978,000	$ 1,447,000
Fund-Based Credit Facility
Line Of Credit Facility [Line Items]
Margin over LIBOR		1.50%	1.50%