Derivative Financial Instruments Outstanding Interest Rate Swap Agreements (Details) - USD ($) $ in Thousands		12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Interest Rate Swap 4 [Member]
Derivatives [Line Items]
Effective Date		Sep. 30, 2015
Expiration Date		Sep. 30, 2016
Interest Rate Swap 4 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Notional Amount		$ 350,000
Pay Fixed Rate, forward interest rate		0.93%
Interest Rate Swap 5 [Member]
Derivatives [Line Items]
Effective Date		Sep. 30, 2016
Expiration Date		Sep. 30, 2026
Interest Rate Swap 5 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Notional Amount		$ 100,000
Pay Fixed Rate, forward interest rate		2.79%
Interest Rate Swap 6 [Member]
Derivatives [Line Items]
Effective Date		Sep. 30, 2016
Expiration Date		Sep. 30, 2026
Interest Rate Swap 6 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Notional Amount		$ 100,000
Pay Fixed Rate, forward interest rate		2.79%
Interest Rate Swap 7 [Member]
Derivatives [Line Items]
Effective Date		Sep. 30, 2016
Expiration Date		Sep. 30, 2026
Interest Rate Swap 7 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Notional Amount		$ 100,000
Pay Fixed Rate, forward interest rate		2.80%
Scenario, Forecast [Member]
Derivatives [Line Items]
Expected settlement date of 10-year forward starting interest rate swaps	Sep. 30, 2016
Fixed rate debt [Member] | Scenario, Forecast [Member]
Derivatives [Line Items]
Probable future borrowings	$ 300,000
Variable rate debt [Member] | Scenario, Forecast [Member]
Derivatives [Line Items]
Probable future borrowings	$ 300,000