Investment Securities - Visa Class B Restricted Shares (Details)	12 Months Ended		96 Months Ended
	Dec. 31, 2016 USD ($) shares	Dec. 31, 2008 USD ($) shares	Dec. 31, 2015 shares	Dec. 31, 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	Dec. 31, 2017
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	$ 22,700,000
Number of shares sold | shares	274,000		0
Estimated amount that was subsequently paid to the buyer						$ 300,000
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	1.6228		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									$ 700,000
Conversion rate							1.6228	1.6298		1.6298	1.6483
Visa | Class B restricted shares
Visa Class B Restricted Shares
Stock received in initial public offering (in shares) | shares		394,000
Visa | Funding Swap (Visa Derivative)
Visa Class B Restricted Shares
Estimated amount that was subsequently paid to the buyer						$ 300,000