Derivatives (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Derivative [Line Items]
Notional amount of derivatives	$ 2,740,984		$ 2,740,984		$ 2,342,791
Ineffective portion of change in derivative fair value	0	$ 0	0	$ 0
Derivative income reflected in the income statement	1,300	1,300	4,400	1,700
Impact to interest income from cash flow hedges	(100)	700	(200)	1,700
Aggregate fair value of derivatives in a net liability position	16,600		16,600
Deposits	21,789	$ 12,590	52,972	$ 36,926
Derivatives Designated as Hedging Instruments [Member]
Derivative [Line Items]
Notional amount of derivatives	1,238,000		1,238,000		1,100,000
Interest Rate Swaps [Member]
Derivative [Line Items]
Deposits	200		600
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member]
Derivative [Line Items]
Notional amount of derivatives	875,000		875,000		$ 1,100,000
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member] | Date One [Member] | Swap Agreement 3, Expires 2019 [Member]
Derivative [Line Items]
Notional amount of derivatives	250,000		$ 250,000
Derivative maturity expiration year			2019
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member] | Date Two [Member] | Swap Agreement 4, Expires 2020 [Member]
Derivative [Line Items]
Notional amount of derivatives	200,000		$ 200,000
Derivative maturity expiration year			2020
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member] | Date Three [Member] | Swap Agreement 5, Expires 2022 [Member]
Derivative [Line Items]
Notional amount of derivatives	$ 425,000		$ 425,000
Derivative maturity expiration year			2022