Derivatives And Risk Management (Interest Rate Swap Agreements) (Details) - Interest Rate Swap [Member] $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) Caontracts	Dec. 31, 2017 USD ($) Caontracts
2018
Derivatives, Fair Value [Line Items]
Number of Interest Rate Derivatives Held | Caontracts	14	14
Derivative, Notional Amount | $	$ 275,000	$ 275,000
Derivative, Maturity Date	Dec. 31, 2018	Dec. 31, 2018
2019
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
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
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
Derivatives, Fair Value [Line Items]
Number of Interest Rate Derivatives Held | Caontracts	5	5
Derivative, Notional Amount | $	$ 60,000	$ 60,000
Derivative, Maturity Date	Dec. 31, 2022	Dec. 31, 2022