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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Related Party Transaction [Line Items]
Expense incurred as result of multiple service agreements					$ 3,009	$ 2,799	$ 8,510	$ 7,587
Period of bear interest at commercial paper rate							30 days
Due to affiliates					9,566		$ 9,566		$ 6,148
Interest expense accrued and/or paid due to intercompany arrangements					41	34	132	66
Interest income accrued and/or received due to intercompany arrangements					0	0	2	6
AEGON USA Realty Advisors, LLC
Related Party Transaction [Line Items]
Expense incurred as result of multiple service agreements					10	57	31	158
AEGON USA Investment Management LLC
Related Party Transaction [Line Items]
Expense incurred as result of multiple service agreements					483	480	1,641	1,410
Transamerica Capital, Inc.
Related Party Transaction [Line Items]
Expense incurred as result of multiple service agreements					5,853	6,063	17,777	17,750
Transamerica Asset Management, Inc.
Related Party Transaction [Line Items]
Expense incurred as result of multiple service agreements						0	0	79
Distribution and shareholder services agreement
Related Party Transaction [Line Items]
Revenue from participation agreement					768	$ 693	2,365	$ 1,891
Transamerica Life Insurance Company
Related Party Transaction [Line Items]
Expense incurred as result of multiple service agreements					$ 211		524
1.10 % Note Payable
Related Party Transaction [Line Items]
Intercompany short-term note payable or receivable							$ 10,000
Interest on intercompany note payable/receivable							1.10%
Intercompany short-term note receivable, date							Oct. 20, 2017
0.83% Note Payable
Related Party Transaction [Line Items]
Intercompany short-term note payable or 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 or receivable	$ 3,000		$ 3,000
Interest on intercompany note payable/receivable			0.90%
Intercompany short-term note receivable, date			Jun. 20, 2017