Derivative Instruments (Narrative) (Details) (USD $)	3 Months Ended				3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012 Interest Rate Exchange Agreement Expiring 2012 [Member]	Mar. 31, 2012 Interest Rate Exchange Agreement Expiring 2011 [Member]	Mar. 31, 2012 Interest Rate Exchange Agreement Expiring 2010 [Member] Y	Mar. 31, 2012 Interest Rate Exchange Agreements Expiring 2011 And 2012 [Member] Y	Dec. 09, 2011 4.2% Senior Notes [Member]	Dec. 06, 2010 4.5 % Senior Notes [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate contract with notional amount			$ 350,000,000	$ 350,000,000	$ 300,000,000
Interest rate on senior notes							4.20%	4.50%
Interest rate exchange agreement expiration date			February 28, 2012	September 30, 2011	December 2010
Notional amount			4,000,000	34,700,000	31,000,000
Settlement amount interest rate contract						38,700,000
Amortization interest expense	369,000	251,000			31,000,000	37,900,000
Term of amortized interest expense, years					10	10
Interest rate					5.80%	5.30%
Other expense	800,000
Amount to be recognized from hedged transactions within 12 months, approximate	$ 7,600,000