Investment Securities - Visa Class B Restricted Shares (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) shares	Dec. 31, 2008 USD ($) shares	Sep. 30, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Oct. 31, 2019 USD ($)	Sep. 30, 2019	Sep. 27, 2019	Jul. 31, 2018 USD ($)	Jul. 05, 2018	Jun. 28, 2018
Visa | Class B restricted shares
Visa Class B Restricted Shares
Historical cost included in the balance sheets		$ 0
Net realized gain related to the sale of stock	$ 22,700
Number of shares sold | shares	274,000
Estimated amount that was subsequently paid to the buyer					$ 300
Shares held | shares			120,000	120,000
Cost basis			$ 0	$ 0
Funding Swap (Visa Derivative) | Class B restricted shares
Visa Class B Restricted Shares
Conversion rate						1.6228	1.6298		1.6298	1.6483
Funding Swap (Visa Derivative) | Visa
Visa Class B Restricted Shares
Estimated amount that was subsequently paid to the buyer					$ 300			$ 700
Visa | Class B restricted shares
Visa Class B Restricted Shares
Stock received in initial public offering (in shares) | shares		394,000