Investments - Schedule of Analysis of Investment Operations (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Net Investment Income [Line Items]
Investment income, gross	$ 232,732	$ 225,590	$ 694,780	$ 670,100
Less investment expense	(3,827)	(3,963)	(11,777)	(11,821)
Net investment income	228,905	221,627	683,003	658,279
Fixed maturities available for sale
Net Investment Income [Line Items]
Investment income, gross	216,154	210,929	647,784	629,242
Policy loans
Net Investment Income [Line Items]
Investment income, gross	10,959	10,490	32,416	30,889
Other long-term investments
Net Investment Income [Line Items]
Investment income, gross	5,105	3,682	12,484	8,656
Short-term investments
Net Investment Income [Line Items]
Investment income, gross	$ 514	$ 489	$ 2,096	$ 1,313

Amount, before investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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