Derivative Instruments - Schedule of Derivative Instruments Reported in Statements of Operations (Detail) - Derivatives Not Designated as Hedging Instruments - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Net investment income \| Equity-indexed options
Derivative Instruments, Gain (Loss) [Line Items]
Gains (Losses) Recognized in Income on Derivatives	$ 23,125	$ 21,778	$ 89,610	$ 7,633
Interest credited to policyholders’ account balances \| Equity-indexed embedded derivative
Derivative Instruments, Gain (Loss) [Line Items]
Gains (Losses) Recognized in Income on Derivatives	$ (36,255)	$ (17,599)	$ (94,411)	$ (4,163)

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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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration