Other Comprehensive Income - Rollforward of AOCI (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2025 USD ($)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	$ 5,172,375
Other comprehensive income (loss) before reclassifications	116,931
Less: Amounts reclassified from AOCI	(12,010)
Balance, end of period	5,172,699
Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(288,162)
Balance, end of period	(159,221)
Net unrealized gains and (losses) on available-for-sale securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(257,878)
Other comprehensive income (loss) before reclassifications	116,931
Less: Amounts reclassified from AOCI	(7,952)
Balance, end of period	(132,995)
Net benefit plan assets and (obligations) recognized in shareholders' equity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(30,284)
Other comprehensive income (loss) before reclassifications	0
Less: Amounts reclassified from AOCI	(4,058)
Balance, end of period	$ (26,226)