Investments - Schedule of Analysis of Investment Operations (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net Investment Income [Line Items]
Investment income, gross									$ 926,504	$ 898,149	$ 862,730
Less investment expense									(16,045)	(15,637)	(14,845)
Net investment income	$ 227,456	$ 228,905	$ 227,425	$ 226,673	$ 224,233	$ 221,627	$ 218,568	$ 218,084	910,459	882,512	847,885
Fixed maturities available for sale
Net Investment Income [Line Items]
Investment income, gross									864,280	843,510	817,213
Policy loans
Net Investment Income [Line Items]
Investment income, gross									43,434	41,359	39,578
Other long-term investments
Net Investment Income [Line Items]
Investment income, gross									16,198	10,638	4,991
Short-term investments
Net Investment Income [Line Items]
Investment income, gross									2,592	2,642	948
Fair value option
Net Investment Income [Line Items]
Net investment income									$ 5,600	$ 3,900	$ 2,500