Loans and Allowance for Credit Losses - Investments by Credit Quality (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	$ 26,860,000	$ 26,309,000
Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	21,190,000	21,267,000
Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	5,616,000	5,008,000
Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	54,000	25,000
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross		9,000
Commercial and Financial Segment
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	24,916,000	24,543,000
Commercial and Financial Segment | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	19,413,000	19,501,000
Commercial and Financial Segment | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	5,449,000	5,008,000
Commercial and Financial Segment | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	54,000	25,000
Commercial and Financial Segment | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross		9,000
Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	1,944,000	1,766,000
Commercial Real Estate | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	1,777,000	1,766,000
Commercial Real Estate | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	167,000	0
Commercial Real Estate | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	0	0
Commercial Real Estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross		0
Domestic
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	19,453,000	20,528,000
Domestic | Commercial and Financial Segment
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	17,509,000
Domestic | Commercial and Financial Segment | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	13,506,000
Domestic | Commercial and Financial Segment | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	3,949,000
Domestic | Commercial and Financial Segment | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	54,000
Domestic | Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	1,944,000	1,766,000
Domestic | Commercial Real Estate | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	1,777,000
Domestic | Commercial Real Estate | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	167,000
Domestic | Loans to investment funds
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	9,696,000	10,270,000
Overdrafts	$ 4,455,000	$ 3,256,000