Derivatives - Derivatives Associated With Interest Rate Risk Management Activities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Customer interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Notional	$ 2,200,080	$ 2,026,753
Assets	7,861	22,097
Liabilities	55,392	18,323
Offsetting upstream interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Notional	2,200,080	2,026,753
Assets	7,904	17,931
Liabilities	8,244	20,720
Customer Interest Rate Contracts Hedging | Hedging Instruments And Hedged Items | Customer interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Notional	1,871,936	1,608,912
Assets	3,792	11,644
Liabilities	50,992	19,780
Customer Interest Rate Contracts Hedging | Hedging Instruments And Hedged Items | Offsetting upstream interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Notional	1,871,936	1,608,912
Assets	14,182	18,473
Liabilities	2,608	11,019
Debt Hedging | Hedging Instruments And Hedged Items | Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Notional	900,000	900,000
Assets	189	371
Debt Hedging | Hedging Instruments And Hedged Items | Term borrowings, Par
Derivative Instruments, Gain (Loss) [Line Items]
Term borrowings	900,000	900,000
Debt Hedging | Hedging Instruments And Hedged Items | Term borrowings, Cumulative fair value hedging adjustments
Derivative Instruments, Gain (Loss) [Line Items]
Term borrowings	(21,782)	(13,472)
Debt Hedging | Hedging Instruments And Hedged Items | Term borrowings, Unamortized premium/(discount) and issuance costs
Derivative Instruments, Gain (Loss) [Line Items]
Term borrowings	(2,699)	(3,910)
Debt Hedging | Hedging Instruments And Hedged Items | Total carrying value
Derivative Instruments, Gain (Loss) [Line Items]
Term borrowings	$ 875,519	$ 882,618