Debt Obligations - Information on Borrowings under Current and Prior Credit Facility and Applicable Interest Rates (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Line of Credit Facility [Abstract]
Maximum amount outstanding during the period	$ 465,599,000	$ 330,473,000	$ 518,607,000	$ 330,473,000
Average daily amount outstanding under the credit facility	$ 426,172,000	$ 171,638,000	$ 449,101,000	$ 132,213,000
Weighted-average interest rate	2.17%	2.01%	2.08%	2.13%