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Debt Obligations - Information on Borrowings under Current and Prior Credit Facility and Applicable Interest Rates (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Debt Disclosure [Abstract]      
Maximum amount outstanding under the credit facility during the period $ 917,895 $ 518,607 $ 606,753
Average daily amount outstanding under the credit facility $ 613,130 $ 458,908 $ 258,815
Weighted-average interest rate 2.70% 2.10% 1.80%