Supplemental Information about Changes to Accumulated Other Comprehensive Income - Summary of Reclassification out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Unrealized investment gains (losses) on available for sale assets:
Realized (gains) losses	$ (11,943)	$ (1,032)	$ (18,426)	$ (14,796)
Amortization of (discount) premium	(228,905)	(221,627)	(683,003)	(658,279)
Total before tax	(247,330)	(220,330)	(704,191)	(660,036)
Income tax benefit (expense)	45,512	41,630	130,370	123,232
Net income	(201,818)	(178,724)	(573,729)	(536,749)
Pension adjustments:
Income tax benefit (expense)	45,512	41,630	130,370	123,232
Reclassification out of Accumulated Other Comprehensive Income
Unrealized investment gains (losses) on available for sale assets:
Net income	(7,032)	2,623	(6,282)	1,279
Reclassification out of Accumulated Other Comprehensive Income \| Unrealized Gains (Losses) on Available for Sale Assets
Unrealized investment gains (losses) on available for sale assets:
Realized (gains) losses	(12,430)	(1,510)	(18,337)	(12,327)
Amortization of (discount) premium	1,410	1,052	4,030	2,612
Total before tax	(11,020)	(458)	(14,307)	(9,715)
Income tax benefit (expense)	2,314	96	3,004	2,040
Net income	(8,706)	(362)	(11,303)	(7,675)
Pension adjustments:
Income tax benefit (expense)	2,314	96	3,004	2,040
Reclassification out of Accumulated Other Comprehensive Income \| Amortization of prior service cost
Pension adjustments:
Pension adjustments	158	119	474	357
Reclassification out of Accumulated Other Comprehensive Income \| Amortization of actuarial (gain) loss
Pension adjustments:
Pension adjustments	1,959	3,659	5,881	10,977
Reclassification out of Accumulated Other Comprehensive Income \| Pension Adjustments
Unrealized investment gains (losses) on available for sale assets:
Total before tax	2,117	3,778	6,355	11,334
Income tax benefit (expense)	(443)	(793)	(1,334)	(2,380)
Net income	1,674	2,985	5,021	8,954
Pension adjustments:
Income tax benefit (expense)	$ (443)	$ (793)	$ (1,334)	$ (2,380)

Amount of realized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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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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Amount of income (expense) related to nonoperating activities, classified as other.

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