Income Taxes - Additional Information (Detail) - USD ($)		3 Months Ended			9 Months Ended			12 Months Ended
	Dec. 22, 2017	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Investments, Owned, Federal Income Tax Note [Line Items]
Federal statutory rate					21.00%		35.00%
Percentage of contract's statutory reserve on date of valuation	92.81%
Increase in tax reserve deductible temporary difference	$ 20,463,000				$ 25,323,000
Offsetting tax reserve net taxable temporary difference against taxable income	$ 20,463,000
Tax reserve against taxable income amortization period	8 years
Decrease in tax reserve deductible temporary difference								$ 4,860,000
Income tax expense (benefit)		$ 0		$ 0	0		$ 575,000
Valuation allowance for deferred tax assets		89,526,000	$ 78,697,000		89,526,000	$ 78,697,000		78,697,000
Unrecognized income tax benefit, net		2,977,000	2,637,000		2,977,000	2,637,000		2,637,000	$ 4,391,000
Unrecognized income tax benefit, gross		14,176,000	12,557,000		14,176,000	12,557,000		12,557,000
Operating loss carry forward		496,635,000	554,553,000		$ 496,635,000	554,553,000		554,553,000
Operating loss carry forward, expiration period					15 years
Alternative minimum tax credit carry forward		2,635,000	4,216,000		$ 2,635,000	4,216,000		4,216,000
Penalties expense related to federal income tax					0		0
Interest expense (income) related to federal income tax		31,000	31,000	$ (5,000)	81,000	81,000	$ 54,000
Interest payable arising from income tax examinations		558,000	477,000		558,000	477,000		477,000
Contributions from (distributions to) capital to Transamerica Corporation					2,727,000			(47,465,000)
Foreign
Investments, Owned, Federal Income Tax Note [Line Items]
Tax credit carry forward		9,889,000	9,831,000		$ 9,889,000	9,831,000		$ 9,831,000
Foreign tax credit carry forward, expiration date in year					10 years			10 years
Capital Loss Carry forward
Investments, Owned, Federal Income Tax Note [Line Items]
Tax credit carry forward		$ 0	$ 0		$ 0	$ 0		$ 0
Earliest Tax Year
Investments, Owned, Federal Income Tax Note [Line Items]
Operating loss carry forward, expiration year					2023
Latest Tax Year
Investments, Owned, Federal Income Tax Note [Line Items]
Operating loss carry forward, expiration year					2031
Latest Tax Year | Foreign
Investments, Owned, Federal Income Tax Note [Line Items]
Foreign tax credit carry forward, expiration year					2028