Financial Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended
	Aug. 31, 2011	Mar. 31, 2012	Dec. 31, 2011
Financial Instruments [Abstract]
Pay-floating interest rate swaps, notional amount	$ 640.0	$ 100.0	$ 200.0
Net settlement proceeds	$ 33.7