Related Party Transactions - Additional Information (Detail) - USD ($) $ in Thousands					3 Months Ended		9 Months Ended
	Aug. 09, 2017	Aug. 04, 2017	Jun. 20, 2017	May 22, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Related Party Transaction [Line Items]
Expense incurred as result of multiple service agreements					$ 2,779	$ 2,799	$ 8,286	$ 7,587
Interest expense accrued and/or paid due to intercompany arrangements					34		66
Interest income accrued and/or received due to intercompany arrangements					0	0	6	27
AEGON USA Realty Advisors, LLC
Related Party Transaction [Line Items]
Expense incurred as result of multiple service agreements					57	48	158	134
AEGON USA Investment Management LLC
Related Party Transaction [Line Items]
Expense incurred as result of multiple service agreements					480	494	1,410	1,498
Transamerica Capital, Inc.
Related Party Transaction [Line Items]
Expense incurred as result of multiple service agreements					6,063	6,212	17,750	18,348
Transamerica Asset Management, Inc.
Related Party Transaction [Line Items]
Expense incurred as result of multiple service agreements					0	82	79	241
Distribution and shareholder services agreement
Related Party Transaction [Line Items]
Revenue from participation agreement					$ 693	$ 593	$ 1,891	$ 1,593
0.83% Note Payable
Related Party Transaction [Line Items]
Intercompany short-term note payable/receivable	$ 20,000	$ 15,000		$ 35,000
Interest on intercompany note payable/receivable				0.83%
Intercompany short-term note receivable, date				May 22, 2017
0.9% Note Payable
Related Party Transaction [Line Items]
Intercompany short-term note payable/receivable			$ 3,000
Interest on intercompany note payable/receivable			0.90%
Intercompany short-term note receivable, date			Jun. 20, 2017