Accumulated Other Comprehensive Income - Reclassifications from AOCI (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on sales of investments	$ 2,167	$ 4,541	$ 2,167	$ 9,708
Net other-than-temporary credit impairment losses	(9,131)	(13,509)	(10,289)	(32,210)
Income before income taxes	118,144	115,156	357,184	277,979
Income taxes	(36)	(139)	(68)	(155)	$ 0
Net income (loss)	$ 118,108	$ 115,017	357,116	277,824
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			4,383	7,777
Net other-than-temporary credit impairment losses			(10,289)	(32,210)
Income before income taxes			(5,906)	(24,433)
Income taxes			0	0
Net income (loss)			$ (5,906)	$ (24,433)