Investment in Securities - Allowance for Credit Losses for Held-to-maturity Securities (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, beginning balance	$ (21,664)
Provision	279
Allowance for credit losses, ending balance	(21,385)
Foreign governments
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, beginning balance	4
Provision	1
Allowance for credit losses, ending balance	5
Corporate debt securities
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, beginning balance	(18,563)
Provision	162
Allowance for credit losses, ending balance	(18,401)
Collateralized debt securities
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, beginning balance	(2,968)
Provision	(18)
Allowance for credit losses, ending balance	(2,986)
Residential mortgage-backed securities
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, beginning balance	(137)
Provision	134
Allowance for credit losses, ending balance	$ (3)