Contract Owners' Account Balances by Mutual Fund Class for Contracts Containing Guaranteed Minimum Death Benefit Provisions (Detail) - Variable Life - GMDB - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners' account balances, guarantee benefit liabilities net	$ 1,383,353	$ 1,326,133
Equity
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners' account balances, guarantee benefit liabilities net	576,367	515,295
Bond
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners' account balances, guarantee benefit liabilities net	117,085	126,634
Balanced
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners' account balances, guarantee benefit liabilities net	599,050	578,880
Money Market
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners' account balances, guarantee benefit liabilities net	$ 90,851	$ 105,324