Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Investments, Owned, Federal Income Tax Note [Line Items]
Federal statutory rate			35.00%	35.00%
Income tax expense (benefit)	$ 0	$ 0	$ 575	$ 0
Valuation allowance for deferred tax assets	126,480		126,480		$ 154,440
Unrecognized income tax benefit, net	4,395		4,395		4,391	$ 3,404
Unrecognized income tax benefit, gross	12,559		12,559		12,547
Operating loss carry forward	554,908		$ 554,908		627,860
Operating loss carry forward, expiration period			15 years
Alternative minimum tax credit carry forward	4,216		$ 4,216		4,216
Interest expense (income) related to federal income tax	(5)	44	54	107
Penalties expense related to federal income tax	1	$ 0	1	$ 0
Interest payable arising from income tax examinations	215		215		161
Return of capital to Transamerica Corporation			(35,720)		(13,381)
Foreign
Investments, Owned, Federal Income Tax Note [Line Items]
Tax credit carry forward	9,853		$ 9,853		$ 9,776
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	$ 84		$ 84		$ 84
Capital loss tax credit carry forward, expiration date in year			5 years		5 years
Capital loss tax credit carry forward, expiration year			2018
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			2027