Investment Securities - Credit Quality Indicators (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	$ 41,165,732	$ 16,592,545
Municipal bonds and notes
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	4,555,237	3,635,194
Municipal bonds and notes | Aaa
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	2,519,992	2,070,311
Municipal bonds and notes | Aa1
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	1,329,903	1,144,500
Municipal bonds and notes | Aa2
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	568,423	420,383
Municipal bonds and notes | Aa3
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	124,724	0
Municipal bonds and notes | Moody's, A1 Rating
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	12,195	0
Corporate bonds
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	51,684	0
Corporate bonds | Aa2
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	27,583	0
Corporate bonds | Aa3
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	$ 24,101	$ 0