Derivatives (Narrative) (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018 USD ($) item	Jun. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Notional amount of derivatives	$ 2,949,325	$ 2,949,325	$ 2,799,296
Derivatives Designated as Hedging Instruments [Member]
Derivative [Line Items]
Notional amount of derivatives	1,358,110	1,358,110	1,358,110
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member]
Derivative [Line Items]
Notional amount of derivatives	875,000	875,000	875,000
Termination fees		10,600
Amortization of accumulated other comprehensive loss on terminated cash flow hedges	$ 1,500	$ 2,500
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	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] | Date One [Member] | $300 Million Swap Agreement [Member]
Derivative [Line Items]
Notional amount of derivatives	425,000	$ 425,000
Derivative maturity expiration year		2022
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member] | Date Two [Member] | $200 Million Swap Agreement [Member]
Derivative [Line Items]
Notional amount of derivatives	350,000	$ 350,000
Derivative maturity expiration year		2023
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member] | Date Three [Member] | Swap Agreement 3, Expires 2024 [Member]
Derivative [Line Items]
Notional amount of derivatives	100,000	$ 100,000
Derivative maturity expiration year		2024
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Fair Value Hedging [Member]
Derivative [Line Items]
Notional amount of derivatives	$ 483,110	$ 483,110	$ 483,110