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Investments - Schedule of Analysis of Investment Operations (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Investment Income [Line Items]
Investment income, gross									$ 944,894	$ 926,504	$ 898,149
Less investment expense									(17,832)	(16,045)	(15,637)
Net investment income	$ 235,071	$ 231,432	$ 231,568	$ 228,991	$ 227,456	$ 228,905	$ 227,425	$ 226,673	927,062	910,459	882,512
Fixed maturities available for sale
Net Investment Income [Line Items]
Investment income, gross									873,352	864,280	843,510
Policy loans
Net Investment Income [Line Items]
Investment income, gross									44,801	43,434	41,359
Other long-term investments
Net Investment Income [Line Items]
Investment income, gross									26,196	16,198	10,638
Short-term investments
Net Investment Income [Line Items]
Investment income, gross									545	2,592	2,642
Fair value option
Net Investment Income [Line Items]
Net investment income									$ 15,300	$ 5,600	$ 3,900