Derivatives (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Notional amount of derivatives	$ 2,799,296	$ 2,342,791
Ineffective portion of change in derivative fair value	0
Aggregate fair value of derivatives in a net liability position	1,800
Collateral obligations for derivative counterparties	4,100
Deposits	76,546	48,934	$ 33,876
Cash Flow Hedge [Member]
Derivative [Line Items]
Impact to interest income from cash flow hedges	(300)	2,300	2,100
Fair Value Hedging [Member]
Derivative [Line Items]
Impact to interest income of the fair value hedges	(800)
Derivatives Designated as Hedging Instruments [Member]
Derivative [Line Items]
Notional amount of derivatives	1,358,110	1,100,000
Interest Rate Swaps [Member]
Derivative [Line Items]
Derivative income reflected in income statement	5,900	5,200	$ 2,700
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member]
Derivative [Line Items]
Notional amount of derivatives	875,000	$ 1,100,000
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member] | Swap Agreement 1, Expires 2019 [Member]
Derivative [Line Items]
Notional amount of derivatives	$ 250,000
Derivative maturity expiration year	2019
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member] | Swap Agreement 2, Expires 2020 [Member]
Derivative [Line Items]
Notional amount of derivatives	$ 200,000
Derivative maturity expiration year	2020
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member] | Swap Agreement 3, Expires 2022 [Member]
Derivative [Line Items]
Notional amount of derivatives	$ 425,000
Derivative maturity expiration year	2022
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Fair Value Hedging [Member]
Derivative [Line Items]
Notional amount of derivatives	$ 483,110