Investment Securities - Credit Quality Indicators (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Municipal bonds and notes, amortized cost	$ 12,982,514	$ 13,842,946
Municipal bonds and notes
Financing Receivable, Credit Quality Indicator [Line Items]
Municipal bonds and notes, amortized cost	2,929,137	$ 1,785,951
Municipal bonds and notes | Aaa
Financing Receivable, Credit Quality Indicator [Line Items]
Municipal bonds and notes, amortized cost	1,732,450
Municipal bonds and notes | Aa1
Financing Receivable, Credit Quality Indicator [Line Items]
Municipal bonds and notes, amortized cost	935,970
Municipal bonds and notes | Aa2
Financing Receivable, Credit Quality Indicator [Line Items]
Municipal bonds and notes, amortized cost	259,767
Municipal bonds and notes | Aa3
Financing Receivable, Credit Quality Indicator [Line Items]
Municipal bonds and notes, amortized cost	$ 950