Derivative Financial Instruments - Free-Standing (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2016 shares	Dec. 31, 2015 shares	Dec. 31, 2014 shares	Dec. 31, 2013 shares	Dec. 31, 2012 shares	Dec. 31, 2011 shares	Dec. 31, 2010 shares	Dec. 31, 2009 shares	Dec. 31, 2008 shares	Jul. 31, 2018 USD ($)	Jun. 28, 2018	Dec. 31, 2017
Visa | Class B restricted shares
Derivative financial instruments
Number of shares sold | shares						274,000	0	0	0	0	0	0	0	0
Funding Swap (Visa Derivative)
Derivative financial instruments
Derivative liability value	$ 1,200,000		$ 1,200,000		$ 2,600,000
Funding Swap (Visa Derivative) | Class B restricted shares
Derivative financial instruments
Conversion rate																1.6298	1.6483
Funding Swap (Visa Derivative) | Visa
Derivative financial instruments
Estimated liability that was subsequently paid to the buyer															$ 700,000
Derivatives Not Designated as Hedging Instruments | Interest rate swaps
Derivative financial instruments
Notional Amount	2,592,239,000		2,592,239,000		$ 2,269,247,000
Net interest expense on derivative	$ 0	$ 100,000	$ 0	$ 300,000
Derivatives Not Designated as Hedging Instruments | Interest rate swaps | Minimum
Derivative financial instruments
Fixed interest rate (as a percent)	2.02%		2.02%		2.02%
Derivatives Not Designated as Hedging Instruments | Interest rate swaps | Maximum
Derivative financial instruments
Fixed interest rate (as a percent)	8.73%		8.73%		5.78%
Derivatives Not Designated as Hedging Instruments | Interest rate swaps | Customer swap program
Derivative financial instruments
Notional Amount	$ 2,600,000,000		$ 2,600,000,000		$ 2,200,000,000
Positive fair value, derivative asset	60,300,000		60,300,000		12,300,000
Negative fair value, derivative liability	0		0		12,000,000.0
Upfront fees on the dealer swap	1,100,000	$ 1,800,000	2,000,000.0		5,000,000.0
Derivative asset value	60,300,000		60,300,000		12,300,000
Derivative liability value	0		0		11,200,000
Derivatives Not Designated as Hedging Instruments | Interest rate swaps | Included in other assets
Derivative financial instruments
Positive fair value, derivative asset	60,281,000		60,281,000		12,305,000
Derivatives Not Designated as Hedging Instruments | Interest rate swaps | Included in other liabilities
Derivative financial instruments
Negative fair value, derivative liability					12,007,000
Derivatives Not Designated as Hedging Instruments | Funding Swap (Visa Derivative)
Derivative financial instruments
Notional Amount	66,840,000		66,840,000		62,039,000
Derivatives Not Designated as Hedging Instruments | Funding Swap (Visa Derivative) | Included in other liabilities
Derivative financial instruments
Negative fair value, derivative liability	$ 1,179,000		$ 1,179,000		$ 2,607,000