Other Comprehensive Income - Rollforward of AOCI (Details) $ in Thousands	6 Months Ended
	Jun. 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	40,873
Less: Amounts reclassified from AOCI	(15,751)
Balance, end of period	4,844,262
Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(481,511)
Balance, end of period	(424,887)
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	42,576
Less: Amounts reclassified from AOCI	(7,849)
Balance, end of period	(358,071)
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	(1,703)
Less: Amounts reclassified from AOCI	(7,902)
Balance, end of period	$ (66,816)