Accumulated Other Comprehensive Income - Reclassifications from AOCI (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on sales of investments	$ 75,854	$ 8,603
Net other-than-temporary credit impairment losses	0	(4,853)
Income (loss) before income taxes	(370,723)	118,147
Income taxes	(32)	0	$ 0
Net income (loss)	(370,755)	118,147
Amounts Reclassified from Accumulated OCI \| Unrealized gains (losses) on available-for-sale securities, net
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on sales of investments	6,641	(14,918)
Net other-than-temporary credit impairment losses	0	(4,853)
Income (loss) before income taxes	6,641	(19,771)
Income taxes	0	0
Net income (loss)	$ 6,641	$ (19,771)

Net gain (loss) resulting from the sales of securitized, pay-through debt securities collateralized by real estate mortgage loans (mortgages).

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