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	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Unrealized investment gains (losses) on available for sale assets:
Realized gains (losses)	$ (5,154)	$ (11,813)	$ (6,483)	$ (13,764)
Amortization of (discount) premium	(227,425)	(218,568)	(454,098)	(436,652)
Total before tax	(228,760)	(226,864)	(456,861)	(439,706)
Income tax benefit (expense)	42,151	42,471	84,858	81,602
Net income	(186,566)	(184,425)	(371,911)	(358,025)
Pension adjustments:
Income tax benefit (expense)	42,151	42,471	84,858	81,602
Reclassification out of Accumulated Other Comprehensive Income
Unrealized investment gains (losses) on available for sale assets:
Net income	1,006	(3,779)	750	(1,344)
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)	(2,237)	(9,431)	(5,907)	(10,817)
Amortization of (discount) premium	1,394	870	2,620	1,560
Total before tax	(843)	(8,561)	(3,287)	(9,257)
Income tax benefit (expense)	177	1,798	690	1,944
Net income	(666)	(6,763)	(2,597)	(7,313)
Pension adjustments:
Income tax benefit (expense)	177	1,798	690	1,944
Reclassification out of Accumulated Other Comprehensive Income \| Amortization of prior service cost
Pension adjustments:
Pension adjustments	158	119	316	238
Reclassification out of Accumulated Other Comprehensive Income \| Amortization of actuarial (gain) loss
Pension adjustments:
Pension adjustments	1,960	3,659	3,922	7,318
Reclassification out of Accumulated Other Comprehensive Income \| Pension Adjustments
Unrealized investment gains (losses) on available for sale assets:
Total before tax	2,118	3,778	4,238	7,556
Income tax benefit (expense)	(446)	(794)	(891)	(1,587)
Net income	1,672	2,984	3,347	5,969
Pension adjustments:
Income tax benefit (expense)	$ (446)	$ (794)	$ (891)	$ (1,587)

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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