Summary of Significant Accounting Policies - Allowance for Credit Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Held-to-maturity Securities	$ 1,641,485	$ 955,542
Threshold Net Book Balance, Individually Evaluated Loans	1,000
Interest Receivable	76,300	93,900
Liability Recorded for Expected Credit Losses on Unfunded Commitments	28,300
Other Assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accrued interest receivable for investment securities	17,000	13,100
Interest Receivable	76,300	93,900
Other Liabilities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Liability Recorded for Expected Credit Losses on Unfunded Commitments	$ 28,300	$ 43,400