Accumulated Other Comprehensive Income - Reclassifications from AOCI (Detail) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on sales of investments	$ 26,380,000	$ (7,526,000)	$ 102,234,000	$ 1,077,000
Net other-than-temporary credit impairment losses			0	(4,853,000)
Income (loss) before income taxes	(54,919,000)	58,915,000	(425,642,000)	177,062,000
Income taxes	(36,000)	(155,000)	(68,000)	(155,000)	$ 0
Net income (loss)	$ (54,955,000)	$ 58,760,000	(425,710,000)	176,907,000
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			33,021,000	(22,187,000)
Net other-than-temporary credit impairment losses			0	(4,853,000)
Income (loss) before income taxes			33,021,000	(27,040,000)
Income taxes			0	0
Net income (loss)			$ 33,021,000	$ (27,040,000)