Derivative Financial Instruments - Free-Standing (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended	96 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 shares	Dec. 31, 2015 shares	Jul. 31, 2018 USD ($)	Jun. 28, 2018	Dec. 31, 2017 USD ($)
Visa | Class B restricted shares
Derivative financial instruments
Number of shares sold | shares					274,000	0
Funding Swap (Visa Derivative) | Class B restricted shares
Derivative financial instruments
Conversion rate					1.6483			1.6298
Funding Swap (Visa Derivative) | Visa
Derivative financial instruments
Estimated liability that was subsequently paid to the buyer							$ 700
Derivatives Not Designated as Hedging Instruments | Interest rate swaps
Derivative financial instruments
Notional Amount	$ 2,166,248		$ 2,166,248						$ 1,820,442
Net interest expense on derivative	$ 100	$ 200	$ 400	$ 700
Derivatives Not Designated as Hedging Instruments | Interest rate swaps | Minimum
Derivative financial instruments
Fixed interest rate (as a percent)	2.02%		2.02%						1.36%
Derivatives Not Designated as Hedging Instruments | Interest rate swaps | Maximum
Derivative financial instruments
Fixed interest rate (as a percent)	5.78%		5.78%						5.33%
Derivatives Not Designated as Hedging Instruments | Interest rate swaps | Customer swap program
Derivative financial instruments
Notional Amount	$ 2,100,000		$ 2,100,000						$ 1,700,000
Upfront fees on the dealer swap	300	$ 1,300	5,300	$ 5,900
Derivative asset value	1,500		1,500						14,700
Derivative liability value	27,800		27,800						11,700
Derivatives Not Designated as Hedging Instruments | Interest rate swaps | Included in other assets
Derivative financial instruments
Positive fair value, derivative asset	1,540		1,540						14,658
Derivatives Not Designated as Hedging Instruments | Interest rate swaps | Included in other liabilities
Derivative financial instruments
Negative fair value, derivative liability	28,388		28,388						13,017
Derivatives Not Designated as Hedging Instruments | Funding Swap (Visa Derivative)
Derivative financial instruments
Notional Amount	59,770		59,770						43,113
Derivatives Not Designated as Hedging Instruments | Funding Swap (Visa Derivative) | Included in other liabilities
Derivative financial instruments
Negative fair value, derivative liability	$ 3,385		$ 3,385						$ 5,439