Investments - Realized Gain (Loss) on Investments (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Gain (Loss) on Securities [Line Items]
Realized gains (losses)	$ 11,943,000	$ 1,032,000	$ 18,426,000	$ 14,796,000
Applicable tax	(2,508,000)	(217,000)	(3,869,000)	(3,107,000)
Realized gains (losses), net of tax	9,435,000	815,000	14,557,000	11,689,000
Realized gains (losses)	11,943,000	1,032,000	18,426,000	14,796,000
Fixed maturities available for sale
Gain (Loss) on Securities [Line Items]
Realized gains (losses)	12,430,000	1,510,000	18,337,000	12,327,000
Exchanges	43,700,000	0	161,000,000.0	128,900,000
Realized gains (losses)	11,800,000		20,100,000	10,400,000
Fair value option—change in fair value
Gain (Loss) on Securities [Line Items]
Realized gains (losses)	(487,000)	(456,000)	197,000	2,405,000
Other investments
Gain (Loss) on Securities [Line Items]
Realized gains (losses)	$ 0	$ (22,000)	$ (108,000)	$ 64,000