Derivative Instruments - Schedule of Derivative Instruments Reported in Statements of Operations (Detail) - Derivatives Not Designated as Hedging Instruments - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Net Investment Income \| Equity-Indexed Options
Derivative Instruments, Gain (Loss) [Line Items]
Gains (Losses) Recognized in Income on Derivatives	$ 66,485	$ (14,145)
Interest Credited to Policyholders' Account Balances \| Embedded Derivative
Derivative Instruments, Gain (Loss) [Line Items]
Gains (Losses) Recognized in Income on Derivatives	$ (58,156)	$ 13,436

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration