Accumulated Other Comprehensive Income - Reclassifications from AOCI (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on sales of investments	$ 17,687	$ 1,596	$ (7,526)	$ 8,603	$ 1,213	$ (6,123)	$ 2,167	$ 0	$ 18,770	$ (2,844)	$ 9,123
Net other-than-temporary credit impairment losses									(4,853)	(21,791)	(61,856)
Income (loss) before income taxes									413,744	411,728	524,776
Income taxes									(193)	(91)	(108)
Net income (loss)	$ 130,318	$ 106,326	$ 58,760	$ 118,147	$ (102,798)	$ 157,321	$ 118,108	$ 239,007	413,551	411,637	$ 524,668
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									(5,796)	6,291
Net other-than-temporary credit impairment losses									(4,853)	(21,791)
Income (loss) before income taxes									(10,649)	(15,500)
Income taxes									0	0
Net income (loss)									$ (10,649)	$ (15,500)