Summary of Changes in Fair Value of Level 3 Liabilities (Assets) (Detail) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2018	Dec. 31, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Liabilities, balance at beginning of period		$ 38,471
Liabilities, balance at end of period		34,811	$ 38,471
Assets, Balance at beginning of period		(29,589)
Assets, balance at end of period		(27,345)	(29,589)
GMWB
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Liabilities, balance at beginning of period	[1]	37,566	54,414
Liabilities, balance at end of period	[1]	33,906	37,566
GMWB | Interest rates
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Liabilities, gain (loss) included in earnings	[2]	(5,683)	5,787
GMWB | Equity markets
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Liabilities, gain (loss) included in earnings	[2]	1,755	(19,803)
GMWB | Other
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Liabilities, gain (loss) included in earnings	[2]	268	(2,832)
GMIB
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, Balance at beginning of period	[1]	(25,325)	(48,166)
Assets, balance at end of period	[1]	(23,259)	(25,325)
GMIB | Interest rates
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, gain (loss) included in earnings	[2]	3,550	(1,841)
GMIB | Equity markets
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, gain (loss) included in earnings	[2]	(575)	24,663
GMIB | Other
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, gain (loss) included in earnings	[2]	(909)	19
SALB
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Liabilities, balance at beginning of period	[1]	905	729
Liabilities, balance at end of period	[1]	$ 905	905
SALB | Equity markets
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Liabilities, gain (loss) included in earnings	[2]		$ 176

[1] GMWB and SALB are recorded as a component of future policy benefits on the Balance Sheets and GMIB reinsurance is recorded as recoverable of ceded GMIB embedded derivatives, at fair value on the Balance Sheets.

[2] Recorded as a component of policy benefits in the Statements of Income (Loss).