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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014		Dec. 31, 2013
Unrealized gains (losses) on available for sale assets:
Realized (gains) losses									$ 8,791	$ (23,548)	[1]	$ (10,668)	[1]
Amortization of (discount) premium	$ (194,319)	$ (193,213)	$ (194,823)	$ (191,596)	$ (190,717)	$ (189,588)	$ (189,930)	$ (188,051)	(773,951)	(758,286)	[1]	(734,650)	[1]
Income tax expense from continuing operations									249,894	256,603	[1]	248,110	[1]
Total after tax	$ (132,956)	$ (145,386)	$ (127,110)	$ (121,648)	$ (146,748)	$ (132,412)	$ (130,923)	$ (132,856)	(527,100)	(542,939)	[1],[2]	(528,472)	[1],[2]
Pension adjustments:
Total after tax									11,517	(14,370)		2,624
Available-for-sale Securities [Member]
Pension adjustments:
Total after tax									2,036	(21,055)		(9,314)
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member]
Pension adjustments:
Pension adjustments reclassified out of accumulated other comprehensive income									377	2,113		2,276
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member]
Pension adjustments:
Pension adjustments reclassified out of accumulated other comprehensive income									14,209	8,172		16,090
Pension Adjustments [Member]
Pension adjustments:
Pension adjustments reclassified out of accumulated other comprehensive income									14,586	10,285		18,366
Tax									(5,105)	(3,600)		(6,428)
Total after tax									9,481	6,685		11,938
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized gains (losses) on available for sale assets [Member]
Unrealized gains (losses) on available for sale assets:
Realized (gains) losses									9,478	(23,771)		(9,606)
Amortization of (discount) premium									(6,346)	(8,621)		(6,569)
Total before tax									3,132	(32,392)		(16,175)
Income tax expense from continuing operations									(1,096)	11,337		6,861
Reclassification out of Accumulated Other Comprehensive Income [Member] | Available-for-sale Securities [Member]
Unrealized gains (losses) on available for sale assets:
Total after tax									$ 2,036	$ (21,055)		$ (9,314)

[1] Certain balances were adjusted to give effect to discontinued operations and the adoption of new accounting guidance as described in Note 1- Significant Accounting Policies under the caption "Low-Income Housing Tax Credit Interests."

[2] Certain balances were adjusted to give effect to discontinued operations as described in Note 1—Significant Accounting Policies.