Accumulated Other Comprehensive Income - Reclassifications from AOCI (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on sales of investments	$ 11,121	$ 3,079	$ 6,631	$ (2,674)	$ 34,285	$ 3,539	$ 9,685	$ 29,565	$ 18,155	$ 77,074	$ 91,709
Net other-than-temporary credit impairment losses	14,780	11,574	20,955	10,678	14,696	17,832	27,101	7,815	57,987	67,444	63,992
Income (loss) before income taxes									552,026	250,350	589,207
Income taxes									(83)	(1)	(2)
Net income (loss)	$ 221,903	$ 172,817	$ 74,127	$ 83,098	$ 115,380	$ (48,259)	$ 116,187	$ 67,042	551,943	250,349	$ 589,205
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									25,145	77,284
Net other-than-temporary credit impairment losses									(57,987)	67,444
Income (loss) before income taxes									(32,842)	9,840
Income taxes									0	0
Net income (loss)									$ (32,842)	$ 9,840