Investment Securities - Visa Class B Restricted Shares (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 shares	Dec. 31, 2014 shares	Dec. 31, 2013 shares	Dec. 31, 2012 shares	Dec. 31, 2011 shares	Dec. 31, 2010 shares	Dec. 31, 2009 shares	Dec. 31, 2008 USD ($) shares	Jun. 30, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Jul. 31, 2018 USD ($)	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
Number of shares sold | shares	274,000	0	0	0	0	0	0	0	0
Shares held | shares										120,000	120,000
Cost basis										$ 0	$ 0
Visa | Class B restricted shares
Visa Class B Restricted Shares
Stock received in initial public offering (in shares) | shares									394,000
Funding Swap (Visa Derivative) | Class B restricted shares
Visa Class B Restricted Shares
Conversion rate													1.6298	1.6483
Funding Swap (Visa Derivative) | Visa
Visa Class B Restricted Shares
Estimated liability that was subsequently paid to the buyer												$ 700