Accumulated Other Comprehensive Income - Reclassifications from AOCI (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on sales of investments	$ 5,167		$ (2,674)
Net other-than-temporary credit impairment losses	(18,701)		(10,678)
Income (loss) before income taxes	162,823		83,127
Income taxes	(16)	$ 0	(29)
Net income (loss)	162,807		83,098
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	5,186		1,612
Net other-than-temporary credit impairment losses	(18,701)		(10,678)
Income (loss) before income taxes	(13,515)		(9,066)
Income taxes	0		0
Net income (loss)	$ (13,515)		$ (9,066)