Investments - Schedule of Net Realized Gains (Losses) by Major Investment Category (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Schedule of Available-for-sale Securities [Line Items]
Total realized gains	$ 2,230	$ 3	$ 3,415	$ 77
Total realized losses	(1,219)	(88)	(1,380)	(389)
Net realized gains and (losses)	1,011	(85)	2,035	(312)
Total realized gains, Fair Value at Sale	46,612	70,570	65,800	234,043
Total realized losses, Fair Value at Sale	11,923	9,004	14,881	65,083
Net realized gain and (losses), Fair Value at Sale	58,535	79,574	80,681	299,126
Fixed Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total realized gains	975	2	968	76
Total realized losses	(140)	(39)	(204)	(242)
Total realized gains, Fair Value at Sale	29,359	70,401	42,387	233,874
Total realized losses, Fair Value at Sale	6,406	7,487	13,089	61,325
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total realized gains	1,255	1	2,447	1
Total realized losses	(1,079)	(49)	(1,176)	(147)
Total realized gains, Fair Value at Sale	17,253	169	23,413	169
Total realized losses, Fair Value at Sale	$ 5,517	$ 1,517	$ 1,792	$ 3,758