Allowance for Expected Credit Losses - Investments by Credit Quality (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	$ 32,379,000	$ 26,309,000
Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	26,890,000	21,267,000
Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	5,413,000	5,008,000
Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	64,000	25,000
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	12,000	9,000
Commercial and Financial Segment
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	30,564,000	24,543,000
Commercial and Financial Segment | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	25,075,000	19,501,000
Commercial and Financial Segment | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	5,413,000	5,008,000
Commercial and Financial Segment | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	64,000	25,000
Commercial and Financial Segment | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	12,000	9,000
Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	1,815,000	1,766,000
Commercial Real Estate | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	1,815,000	1,766,000
Commercial Real Estate | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	0	0
Commercial Real Estate | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments		0
Commercial Real Estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	0	0
Geographic Distribution, Domestic | Commercial and Financial Segment
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	21,145,000
Geographic Distribution, Domestic | Commercial and Financial Segment | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	17,547,000
Geographic Distribution, Domestic | Commercial and Financial Segment | Speculative
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	3,524,000
Geographic Distribution, Domestic | Commercial and Financial Segment | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	64,000
Geographic Distribution, Domestic | Commercial and Financial Segment | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	10,000
Geographic Distribution, Domestic | Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	1,815,000
Geographic Distribution, Domestic | Commercial Real Estate | Investment grade
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	1,815,000
Geographic Distribution, Domestic | Loans to investment funds
Financing Receivable, Credit Quality Indicator [Line Items]
Overdrafts	8,685,000	$ 3,256,000
Impaired Loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	25,000
Impaired Loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total investments	$ 10,000