Derivative Financial Instruments (Fixed-For-Variable Interest Rates) (Details) (Fixed-For-Variable [Member], USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2011
Interest Swap Due July 2016 [Member]
Notional Amount	$ 400,000
Weighted Average Fixed Interest Rate	2.87%
Termination Date	Jul. 01, 2016
Interest Swap Due March 2017 [Member]
Notional Amount	$ 70,000
Weighted Average Fixed Interest Rate	3.23%
Termination Date	Mar. 01, 2017