Loans - Net Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ (97,000)	$ (74,000)	$ (70,000)	$ (67,000)
Loans, net of allowance	32,282,000	26,235,000
Geographic Distribution, Domestic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	22,960,000	20,528,000
Geographic Distribution, Domestic | Loans to investment funds
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	17,077,000	14,546,000
Overdrafts	8,685,000	3,256,000
Geographic Distribution, Domestic | Senior secured bank loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	3,194,000	3,342,000
Geographic Distribution, Domestic | Loans to municipalities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	850,000	848,000
Geographic Distribution, Domestic | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	24,000	26,000
Geographic Distribution, Domestic | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	1,815,000	1,766,000
Foreign
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	9,419,000	5,781,000
Foreign | Loans to investment funds
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	8,181,000	4,662,000
Foreign | Senior secured bank loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	1,211,000	1,119,000
Foreign | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	27,000	0
Loans, Excluding Unfunded Commitments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, gross	32,379,000	26,309,000
Allowance for loan losses	(97,000)	(74,000)
Loans, net of allowance	$ 32,282,000	$ 26,235,000