Loans and Allowance for Credit Losses - Investments by Credit Quality (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	$ 27,035,000	$ 26,309,000
Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	21,850,000	21,267,000
Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	5,000,000	5,008,000
Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	126,000	25,000
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	35,000	9,000
Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	24,000
Commercial and Financial Segment
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	25,091,000	24,543,000
Commercial and Financial Segment | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	20,095,000	19,501,000
Commercial and Financial Segment | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	4,811,000	5,008,000
Commercial and Financial Segment | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	126,000	25,000
Commercial and Financial Segment | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	35,000	9,000
Commercial and Financial Segment | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	24,000
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	1,755,000	1,766,000
Commercial Real Estate | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	189,000	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	0
Commercial Real Estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	0
Domestic
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	19,952,000	20,528,000
Domestic | Commercial and Financial Segment
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	18,008,000
Domestic | Commercial and Financial Segment | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	14,360,000
Domestic | Commercial and Financial Segment | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	3,485,000
Domestic | Commercial and Financial Segment | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	126,000
Domestic | Commercial and Financial Segment | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	35,000
Domestic | Commercial and Financial Segment | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	2,000
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	1,755,000
Domestic | Commercial Real Estate | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	189,000
Domestic | Loans to investment funds
Financing Receivable, Credit Quality Indicator [Line Items]
Loans, gross	10,012,000	10,270,000
Overdrafts	$ 3,793,000	$ 3,256,000