Investment Securities - Credit Quality Indicators (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity debt securities, at amortized cost	$ 16,592,545	$ 13,842,946
Municipal bonds and notes
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity debt securities, at amortized cost	3,635,194	$ 1,785,951
Aaa | Municipal bonds and notes
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity debt securities, at amortized cost	2,070,311
Aa1 | Municipal bonds and notes
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity debt securities, at amortized cost	1,144,500
Aa2 | Municipal bonds and notes
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity debt securities, at amortized cost	$ 420,383