Investment Securities - Amortized cost and fair value by contractual maturity (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 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	36,471,000		36,471,000
Due after five years through ten years	444,858,000		444,858,000
Due after ten years	1,805,613,000		1,805,613,000
Total	2,301,307,000		2,301,307,000		$ 2,487,440,000
Fair Value
Due in one year or less	14,222,000		14,222,000
Due after one year through five years	33,648,000		33,648,000
Due after five years through ten years	393,780,000		393,780,000
Due after ten years	1,522,096,000		1,522,096,000
Fair Value	1,963,746,000		1,963,746,000		2,084,736,000
Amortized Cost
Due in one year or less	84,976,000		84,976,000
Due after one year through five years	341,428,000		341,428,000
Due after five years through ten years	1,217,855,000		1,217,855,000
Due after ten years	3,559,939,000		3,559,939,000
Amortized Cost	5,204,198,000		5,204,198,000		5,561,006,000
Fair Value
Due in one year or less	84,297,000		84,297,000
Due after one year through five years	327,521,000		327,521,000
Due after five years through ten years	1,084,899,000		1,084,899,000
Due after ten years	3,067,646,000		3,067,646,000
Fair Value	4,564,363,000		4,564,363,000		$ 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