Accumulated Other Comprehensive Income - Reclassifications from AOCI (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on sales of investments	$ (586)	$ 1	$ 4,541	$ 5,167	$ 11,121	$ 3,079	$ 6,631	$ (2,674)	$ 9,123	$ 18,155	$ 77,074
Net other-than-temporary credit impairment losses	(18,179)	(11,468)	(13,509)	(18,701)	(14,780)	(11,574)	(20,955)	(10,678)	(61,856)	(57,987)	(67,444)
Income before income taxes									524,776	552,026	250,350
Income taxes									(108)	(83)	(1)
Net income (loss)	$ 107,608	$ 139,232	$ 115,017	$ 162,807	$ 221,903	$ 172,817	$ 74,127	$ 83,098	524,668	551,943	$ 250,349
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									7,278	25,145
Net other-than-temporary credit impairment losses									(61,856)	(57,987)
Income before income taxes									(54,578)	(32,842)
Income taxes									0	0
Net income (loss)									$ (54,578)	$ (32,842)