Other Comprehensive Income (Schedule of Accumulated Other Comprehensive Income, Net of Tax) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ (1,348,294)
Net other comprehensive income (loss) during period	229,075	$ (1,695,612)	$ (165,652)
Ending Balance	(1,119,219)	(1,348,294)
Available-for-sale Securities [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(1,313,791)	380,209	563,801
Other comprehensive income (loss) before reclassification	219,049	(1,694,000)	(183,537)
Reclassification of amounts included in net income	(52)	0	(55)
Net other comprehensive income (loss) during period	218,997	(1,694,000)	(183,592)
Ending Balance	(1,094,794)	(1,313,791)	380,209
Defined Benefit Plans [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(34,503)	(32,891)	(50,831)
Other comprehensive income (loss) before reclassification	7,330	(3,954)	13,108
Reclassification of amounts included in net income	2,748	2,342	4,832
Net other comprehensive income (loss) during period	10,078	(1,612)	17,940
Ending Balance	(24,425)	(34,503)	(32,891)
AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(1,348,294)	347,318	512,970
Other comprehensive income (loss) before reclassification	226,379	(1,697,954)	(170,429)
Reclassification of amounts included in net income	2,696	2,342	4,777
Net other comprehensive income (loss) during period	229,075	(1,695,612)	(165,652)
Ending Balance	$ (1,119,219)	$ (1,348,294)	$ 347,318