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, 2015	Jun. 30, 2014		Jun. 30, 2015	Jun. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Realized (gains) losses	$ (2,613)	$ (577)	[1]	$ (2,732)	$ (17,196)	[1]
Amortization of (discount) premium	(194,823)	(189,930)	[1]	(386,419)	(377,981)	[1]
Income taxes	61,278	62,888	[1]	120,062	127,570	[1]
Net income	(127,110)	(130,923)		(248,758)	(263,779)	[1]
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net income	374	(208)		1,575	(10,784)
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	(1,479)	(577)		(1,598)	(17,196)
Amortization of (discount) premium	(1,599)	(2,317)		(3,271)	(4,544)
Total before tax	(3,078)	(2,894)		(4,869)	(21,740)
Income taxes	1,077	1,013		1,704	7,610
Net income	(2,001)	(1,881)		(3,165)	(14,130)
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	82	528		163	1,056
Amortization of actuarial gain (loss)	3,571	2,046		7,128	4,092
Total before tax	3,653	2,574		7,291	5,148
Income taxes	(1,278)	(901)		(2,551)	(1,802)
Net income	$ 2,375	$ 1,673		$ 4,740	$ 3,346

[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