Supplemental Information about Changes to Accumulated Other Comprehensive Income - Summary of Reclassification out of Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Realized (gains) losses	$ (119)	$ (16,619)	[1]
Amortization of (discount) premium	(191,596)	(188,051)	[1]
Income taxes	58,784	64,682	[1]
Net income	(121,648)	(132,856)	[1]
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net income	1,201	(10,576)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Realized (gains) losses	(119)	(16,619)
Amortization of (discount) premium	(1,672)	(2,227)
Total before tax	(1,791)	(18,846)
Income taxes	627	6,597
Net income	(1,164)	(12,249)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension adjustments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of prior service cost	81	528
Amortization of actuarial gain (loss)	3,557	2,046
Total before tax	3,638	2,574
Income taxes	(1,273)	(901)
Net income	$ 2,365	$ 1,673

[1] Certain balances were retrospectively adjusted to give effect to the adoption of new accounting guidance as described in Note G- Adoption of New Accounting Standards