Investments - Unrealized Gain (Loss) on Investments (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Net change in unrealized gains (losses)	$ 921,421	$ 553,801	$ (1,166,032)
Fixed maturities available for sale
Gain (Loss) on Investments [Line Items]
Net change in unrealized gains (losses)	916,413	551,658	(1,163,295)
Other investments
Gain (Loss) on Investments [Line Items]
Net change in unrealized gains (losses)	$ 5,008	$ 2,143	$ (2,737)