Other Comprehensive Income - Rollforward of AOCI (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2023 USD ($)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	$ 4,642,740
Other comprehensive income (loss) before reclassifications	(35,449)
Less: Amounts reclassified from AOCI	(20,065)
Balance, end of period	4,854,705
Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(481,511)
Balance, end of period	(496,895)
Net unrealized gains and (losses) on available-for-sale securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(408,496)
Other comprehensive income (loss) before reclassifications	(35,082)
Less: Amounts reclassified from AOCI	(10,286)
Balance, end of period	(433,292)
Net benefit plan assets and (obligations) recognized in shareholders' equity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(73,015)
Other comprehensive income (loss) before reclassifications	(367)
Less: Amounts reclassified from AOCI	(9,779)
Balance, end of period	$ (63,603)