This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting.

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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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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								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrealized gains (losses) on available for sale assets:
Realized (gains) losses									$ (23,611)	$ 10,683	$ 8,791
Amortization of (discount) premium	$ (212,955)	$ (213,872)	$ (212,776)	$ (208,282)	$ (205,488)	$ (202,720)	$ (201,642)	$ (197,053)	(847,885)	(806,903)	(773,951)
Income before income taxes	(216,371)	(220,610)	(201,926)	(191,741)	(175,982)	(201,461)	(199,344)	(195,448)	(830,648)	(772,235)	(766,187)
Income tax expense from continuing operations									(627,615)	232,645	249,894
Net income	$ (1,027,346)	$ (153,334)	$ (140,273)	$ (133,541)	$ (135,448)	$ (151,869)	$ (138,429)	$ (124,033)	(1,454,494)	(549,779)	(527,100)
Pension adjustments:
Total reclassifications (after tax)									(14,668)	10,574	11,517
Unrealized Gains (Losses) on Available for Sale Assets
Pension adjustments:
Total reclassifications (after tax)									(22,751)	3,965	2,036
Pension Adjustments, Amortization of Prior Service Cost
Pension adjustments:
Total before tax									476	477	377
Pension Adjustments, Amortization of Actuarial (Gain) Loss
Pension adjustments:
Total before tax									11,960	9,691	14,209
Pension Adjustments
Pension adjustments:
Total before tax									12,436	10,168	14,586
Tax									(4,353)	(3,559)	(5,105)
Total reclassifications (after tax)									8,083	6,609	9,481
Reclassification out of Accumulated Other Comprehensive Income \| Unrealized Gains (Losses) on Available for Sale Assets
Unrealized gains (losses) on available for sale assets:
Realized (gains) losses									(34,954)	10,285	9,478
Amortization of (discount) premium									(47)	(4,185)	(6,346)
Income before income taxes									(35,001)	6,100	3,132
Income tax expense from continuing operations									12,250	(2,135)	(1,096)
Net income									$ (22,751)	$ 3,965	$ 2,036