Summary of Change in Fair Value of Level 3 Fixed Maturity AFS Securities (Detail) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period		$ 29,589
Balance at end of period		27,345	$ 29,589
Fixed maturity AFS securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	[1]	4,264	9,215
Change in unrealized gains (losses)	[2]	(121)	254
Purchases			4,000
Sales		(57)	(5,242)
Transfers into Level 3			3,000
Transfers out of Level 3			(6,965)
Changes in valuation	[3]		1
Net realized investment gains	[4]		1
Balance at end of period	[1]	$ 4,086	$ 4,264

[1] Recorded as a component of fixed maturity AFS securities on the Balance Sheets.

[2] Recorded as a component of other comprehensive income (loss) (“OCI”) in net unrealized holding gains (losses) on AFS securities arising during the period.

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

[4] Recorded as a component of net realized investment gains (losses) on securities in the Statements of Income (Loss).