Derivative Financial Instruments - Free-Standing (Details) $ in Thousands	3 Months Ended		12 Months Ended	96 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2016 shares	Dec. 31, 2015 shares	Dec. 31, 2018 USD ($)	Jul. 31, 2018 USD ($)	Jun. 28, 2018	Dec. 31, 2017
Visa | Class B restricted shares
Derivative financial instruments
Number of shares sold | shares			274,000	0
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
Derivatives Not Designated as Hedging Instruments | Interest rate swaps
Derivative financial instruments
Notional Amount	$ 2,334,559				$ 2,269,247
Net interest expense on derivative	$ 0	$ 200
Derivatives Not Designated as Hedging Instruments | Interest rate swaps | Minimum
Derivative financial instruments
Fixed interest rate (as a percent)	2.02%				2.02%
Derivatives Not Designated as Hedging Instruments | Interest rate swaps | Maximum
Derivative financial instruments
Fixed interest rate (as a percent)	5.78%				5.78%
Derivatives Not Designated as Hedging Instruments | Interest rate swaps | Customer swap program
Derivative financial instruments
Notional Amount					$ 2,200,000
Upfront fees on the dealer swap	$ 800	$ 3,200
Derivative asset value	29,000				12,300
Derivative liability value	1,900				11,200
Derivatives Not Designated as Hedging Instruments | Interest rate swaps | Included in other assets
Derivative financial instruments
Positive fair value, derivative asset	29,034				12,305
Derivatives Not Designated as Hedging Instruments | Interest rate swaps | Included in other liabilities
Derivative financial instruments
Negative fair value, derivative liability	1,897				12,007
Derivatives Not Designated as Hedging Instruments | Funding Swap (Visa Derivative)
Derivative financial instruments
Notional Amount	64,395				62,039
Derivatives Not Designated as Hedging Instruments | Funding Swap (Visa Derivative) | Included in other liabilities
Derivative financial instruments
Negative fair value, derivative liability	$ 1,839				$ 2,607