Regulatory Capital - Adjusted Transition Amounts (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Jan. 01, 2020
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained earnings	$ 3,884,694	$ 3,624,859
Allowance for credit losses	1,350,058	1,339,772	$ 1,479,306	$ 1,357,075
Liability for unfunded commitments	32,034	112,962	$ 32,720	124,924
Adjusted Phase-In Amounts Remaining
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained earnings	209,087				$ 836,351
Allowance for credit losses	259,537				1,038,145
Liability for unfunded commitments	26,094				104,377
Deferred tax asset	76,544				$ 306,171
Adjustments
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained earnings	(209,088)	(209,088)		(209,088)		$ 953,000
Allowance for credit losses	(259,536)	(259,536)		(259,536)		1,100,000
Liability for unfunded commitments	(26,095)	(26,094)		(26,094)		116,000
Deferred tax asset	$ (76,543)	$ (76,542)		$ (76,542)		$ 306,000