Derivatives And Risk Management (Interest Rate Swap Agreements) (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) Caontracts	Dec. 31, 2017 USD ($) Caontracts
Derivatives, Fair Value [Line Items]
Payments for (proceeds from) settlement of derivative instruments	$ (26,600)
2018 | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Number of interest rate swaps settled | Caontracts	14
Number of Interest Rate Derivatives Held | Caontracts		14
Derivative, Notional Amount		$ 275,000
Derivative, Maturity Date		Dec. 31, 2018
Settled derivative notional amount	$ 275,000
2019 | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Number of Interest Rate Derivatives Held | Caontracts	6	6
Derivative, Notional Amount	$ 70,000	$ 70,000
Derivative, Maturity Date	Dec. 31, 2019	Dec. 31, 2019
2020 | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Number of Interest Rate Derivatives Held | Caontracts	4	3
Derivative, Notional Amount	$ 40,000	$ 30,000
Derivative, Maturity Date	Dec. 31, 2020	Dec. 31, 2020
2021 | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Number of Interest Rate Derivatives Held | Caontracts	1	1
Derivative, Notional Amount	$ 15,000	$ 15,000
Derivative, Maturity Date	Dec. 31, 2021	Dec. 31, 2021
2022 | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Number of Interest Rate Derivatives Held | Caontracts	6	5
Derivative, Notional Amount	$ 70,000	$ 60,000
Derivative, Maturity Date	Dec. 31, 2022	Dec. 31, 2022