Derivatives (Narrative) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) item	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Derivative [Line Items]
Notional amount of derivatives	$ 3,141,363		$ 3,024,565
Fair value liability	6,900		7,300
Derivatives Designated as Hedging Instruments [Member]
Derivative [Line Items]
Notional amount of derivatives	1,363,110		1,358,110
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member]
Derivative [Line Items]
Notional amount of derivatives	975,000		$ 875,000
Termination fees	10,600
Amortization of accumulated other comprehensive loss on terminated cash flow hedges	$ 1,400	$ 1,000
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member] | Swap Agreement One To Five [Member]
Derivative [Line Items]
Number of interest rate swap agreements | item	5
Number of interest rate swap terminated | item	5
Notional amount of derivatives	$ 450,000	$ 450,000
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member] | Swap Agreement 1, Expires 2022 [Member]
Derivative [Line Items]
Notional amount of derivatives	$ 475,000
Derivative maturity expiration year	2022
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member] | Swap Agreement 2, Expires 2023 [Member]
Derivative [Line Items]
Notional amount of derivatives	$ 350,000
Derivative maturity expiration year	2023
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member] | Swap Agreement 3, Expires 2024 [Member]
Derivative [Line Items]
Notional amount of derivatives	$ 100,000
Derivative maturity expiration year	2024