Supplemental Information about Changes to Accumulated Other Comprehensive Income - Schedule of Reclassifications (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Unrealized investment (gains) losses on available for sale assets:
Realized (gains) losses	$ 4,987	$ 2,192	$ 23,476	$ 26,580
Amortization of (discount) premium	(286,013)	(284,964)	(848,796)	(853,178)
Other operating expense	111,562	104,874	328,601	297,196
Total before tax	(477,102)	(376,958)	(1,106,482)	(1,014,204)
Tax	89,259	73,964	211,327	198,638
Total after-tax	(387,843)	(302,994)	(895,155)	(815,566)
Reclassification out of Accumulated Other Comprehensive Income
Unrealized investment (gains) losses on available for sale assets:
Total after-tax	7,897	(1,810)	13,734	2,318
Reclassification out of Accumulated Other Comprehensive Income | Unrealized investment (gains) losses on available for sale assets:
Unrealized investment (gains) losses on available for sale assets:
Realized (gains) losses	11,746	(257)	23,755	9,732
Amortization of (discount) premium	(1,813)	(2,151)	(6,560)	(7,152)
Total before tax	9,933	(2,408)	17,195	2,580
Tax	(2,086)	505	(3,611)	(542)
Total after-tax	7,847	(1,903)	13,584	2,038
Reclassification out of Accumulated Other Comprehensive Income | Pension adjustments:
Unrealized investment (gains) losses on available for sale assets:
Total before tax	64	118	191	354
Tax	(14)	(25)	(41)	(74)
Total after-tax	50	93	150	280
Reclassification out of Accumulated Other Comprehensive Income | Amortization of prior service cost
Unrealized investment (gains) losses on available for sale assets:
Other operating expense	292	265	876	803
Reclassification out of Accumulated Other Comprehensive Income | Amortization of actuarial (gain) loss
Unrealized investment (gains) losses on available for sale assets:
Other operating expense	$ (228)	$ (147)	$ (685)	$ (449)