Regulatory Capital - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
	Jan. 01, 2020	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Allowance for credit losses		$ 1,350,058	$ 1,479,306	$ 1,339,772	$ 1,357,075
Liability for unfunded commitments		32,034	32,720	112,962	124,924
Retained earnings		3,884,694		3,624,859
Unrealized gain (loss)		(115,000)		(115,000)
Unrealized gain (loss), tax benefit		(37,000)		(37,000)
Dividends		160,000	$ 0
Adjustments
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Period of deferment	2 years
Percentage amount deferred per year	25.00%
Percentage amount phased in during year	25.00%
Allowance for credit losses	$ 1,100,000	(259,536)		(259,536)	(259,536)
Liability for unfunded commitments	116,000	(26,095)		(26,094)	(26,094)
Deferred tax asset	306,000	(76,543)		(76,542)	(76,542)
Retained earnings	$ 953,000	$ (209,088)		$ (209,088)	$ (209,088)