Summary of Significant Accounting Policies - Allowance for Credit Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investment Securities - Held to Maturity	$ 2,533,713	$ 2,683,241
Financing Receivable, Threshold Loss on Modifications Estimated	1,000
Accrued interest receivable (AIR)	$ 123,100	$ 105,400
Financing Receivable, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration]	Other Assets.	Other Assets.
Liability Recorded for Expected Credit Losses on Unfunded Commitments	$ 62,300
Other Assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accrued interest receivable for investment securities	25,600	$ 28,200
Other Liabilities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Liability Recorded for Expected Credit Losses on Unfunded Commitments	$ 62,300	$ 67,200