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		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Unrealized investment gains (losses) on available for sale assets:
Realized (gains) losses	$ (5,140)	$ 1,483	$ (7,872)	$ (15,713)
Amortization of (discount) premium	(193,213)	(189,588)	(579,632)	(567,569)
Tax	71,358	64,639	191,420	192,209
Net income	(145,386)	(132,412)	(394,144)	(396,191)
Pension adjustments:
Total reclassifications (after tax)	(2,252)	958	(677)	(9,826)
Available for Sale Assets [Member]
Pension adjustments:
Total reclassifications (after tax)	(4,621)	(711)	(7,786)	(14,841)
Amortization of prior service cost
Pension adjustments:
Pension adjustments reclassified out of accumulated other comprehensive income	81	529	244	1,586
Amortization of actuarial gain (loss)
Pension adjustments:
Pension adjustments reclassified out of accumulated other comprehensive income	3,563	2,038	10,691	6,130
Pension adjustments [Member]
Pension adjustments:
Pension adjustments reclassified out of accumulated other comprehensive income	3,644	2,567	10,935	7,716
Tax	(1,275)	(898)	(3,826)	(2,701)
Total reclassifications (after tax)	2,369	1,669	7,109	5,015
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized investment gains (losses) on available for sale assets Member]
Unrealized investment gains (losses) on available for sale assets:
Realized (gains) losses	(5,540)	1,261	(7,138)	(15,935)
Amortization of (discount) premium	(1,569)	(2,354)	(4,840)	(6,898)
Total before tax	(7,109)	(1,093)	(11,978)	(22,833)
Tax	2,488	382	4,192	7,992
Reclassification out of Accumulated Other Comprehensive Income [Member] | Available for Sale Assets [Member]
Unrealized investment gains (losses) on available for sale assets:
Net income	$ (4,621)	$ (711)	$ (7,786)	$ (14,841)