Debt Securities - Debt Securities in an Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2024 USD ($) security	Dec. 31, 2023 USD ($) security
Available-for-sale
Number of Securities | security	727	713
Less than 12 Months, Fair Value	$ 57,282	$ 29,403
Less than 12 Months, Unrealized Loss	(1,803)	(370)
12 Months or More, Fair Value	4,528,569	4,679,701
12 Months or More, Unrealized Loss	(494,100)	(498,005)
Total, Fair Value	4,585,851	4,709,104
Total, Unrealized Loss	$ (495,903)	$ (498,375)
U.S. government and federal agency
Available-for-sale
Number of Securities | security	56	57
Less than 12 Months, Fair Value	$ 2,512	$ 3,702
Less than 12 Months, Unrealized Loss	(42)	(56)
12 Months or More, Fair Value	449,659	448,909
12 Months or More, Unrealized Loss	(29,915)	(29,613)
Total, Fair Value	452,171	452,611
Total, Unrealized Loss	$ (29,957)	$ (29,669)
U.S. government sponsored enterprises
Available-for-sale
Number of Securities | security	16	14
Less than 12 Months, Fair Value	$ 6,931	$ 0
Less than 12 Months, Unrealized Loss	(5)	0
12 Months or More, Fair Value	299,695	299,220
12 Months or More, Unrealized Loss	(22,761)	(22,774)
Total, Fair Value	306,626	299,220
Total, Unrealized Loss	$ (22,766)	$ (22,774)
State and local governments
Available-for-sale
Number of Securities | security	99	85
Less than 12 Months, Fair Value	$ 14,909	$ 3,039
Less than 12 Months, Unrealized Loss	(43)	(2)
12 Months or More, Fair Value	64,440	64,645
12 Months or More, Unrealized Loss	(3,739)	(3,271)
Total, Fair Value	79,349	67,684
Total, Unrealized Loss	$ (3,782)	$ (3,273)
Corporate bonds
Available-for-sale
Number of Securities | security	2	4
Less than 12 Months, Fair Value	$ 0	$ 0
Less than 12 Months, Unrealized Loss	0	0
12 Months or More, Fair Value	13,362	23,262
12 Months or More, Unrealized Loss	(675)	(756)
Total, Fair Value	13,362	23,262
Total, Unrealized Loss	$ (675)	$ (756)
Residential mortgage-backed securities
Available-for-sale
Number of Securities | security	402	402
Less than 12 Months, Fair Value	$ 1,040	$ 1,430
Less than 12 Months, Unrealized Loss	(28)	(44)
12 Months or More, Fair Value	2,677,271	2,809,482
12 Months or More, Unrealized Loss	(350,672)	(355,289)
Total, Fair Value	2,678,311	2,810,912
Total, Unrealized Loss	$ (350,700)	$ (355,333)
Commercial mortgage-backed securities
Available-for-sale
Number of Securities | security	152	151
Less than 12 Months, Fair Value	$ 31,890	$ 21,232
Less than 12 Months, Unrealized Loss	(1,685)	(268)
12 Months or More, Fair Value	1,024,142	1,034,183
12 Months or More, Unrealized Loss	(86,338)	(86,302)
Total, Fair Value	1,056,032	1,055,415
Total, Unrealized Loss	$ (88,023)	$ (86,570)