Debt Debt (Schedule of Unsecured Term Loans) (Details) - Private Placement [Member] - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Unsecured Debt	$ 400,000	$ 400,000
January 2023 [Member]
Unsecured Debt	$ 125,000
Debt Instrument, Interest Rate, Stated Percentage	3.99%
November 2023 [Member]
Unsecured Debt	$ 50,000
Debt Instrument, Interest Rate, Stated Percentage	3.99%
September 2024 [Member]
Unsecured Debt	$ 75,000
Debt Instrument, Interest Rate, Stated Percentage	3.93%
November 2025 [Member]
Unsecured Debt	$ 50,000
Debt Instrument, Interest Rate, Stated Percentage	4.33%
January 2027 [Member]
Unsecured Debt	$ 100,000
Debt Instrument, Interest Rate, Stated Percentage	4.51%