Investment Securities - Credit Quality Indicators (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Municipal bonds and notes, amortized cost	$ 13,574,519	$ 13,842,946
Municipal bonds and notes
Financing Receivable, Credit Quality Indicator [Line Items]
Municipal bonds and notes, amortized cost	2,160,341	$ 1,785,951
Municipal bonds and notes | Aaa
Financing Receivable, Credit Quality Indicator [Line Items]
Municipal bonds and notes, amortized cost	1,458,546
Municipal bonds and notes | Aa1
Financing Receivable, Credit Quality Indicator [Line Items]
Municipal bonds and notes, amortized cost	477,068
Municipal bonds and notes | Aa2
Financing Receivable, Credit Quality Indicator [Line Items]
Municipal bonds and notes, amortized cost	223,357
Municipal bonds and notes | Aa3
Financing Receivable, Credit Quality Indicator [Line Items]
Municipal bonds and notes, amortized cost	$ 1,370