Investment Securities - Amortized cost and fair value by contractual maturity (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Amortized Cost
Due in one year or less	$ 14,365,000		$ 14,365,000
Due after one year through five years	36,511,000		36,511,000
Due after five years through ten years	456,439,000		456,439,000
Due after ten years	1,841,213,000		1,841,213,000
Total	2,348,528,000		2,348,528,000		$ 2,487,440,000
Fair Value
Due in one year or less	14,057,000		14,057,000
Due after one year through five years	32,483,000		32,483,000
Due after five years through ten years	389,368,000		389,368,000
Due after ten years	1,485,033,000		1,485,033,000
Fair Value	1,920,941,000		1,920,941,000		2,084,736,000
Amortized Cost
Due in one year or less	143,274,000		143,274,000
Due after one year through five years	306,612,000		306,612,000
Due after five years through ten years	1,239,040,000		1,239,040,000
Due after ten years	3,635,694,000		3,635,694,000
Amortized Cost	5,324,620,000		5,324,620,000		5,561,006,000
Fair Value
Due in one year or less	141,624,000		141,624,000
Due after one year through five years	285,442,000		285,442,000
Due after five years through ten years	1,063,459,000		1,063,459,000
Due after ten years	3,007,739,000		3,007,739,000
Fair Value	4,498,264,000		4,498,264,000		$ 4,784,388,000
Information with respect to sales of held-to-maturity
Proceeds from sales of investment securities held to maturity	0	$ 0	0	$ 0
Information with respect to sales of available-for-sale securities
Gross gains		1,300,000
Gross losses				(1,300,000)
Sold securities	0		0
Realized gains or losses	$ 0	$ 45,000	$ 0	$ 45,000