Disclosures About Fair Value Measurements (Narrative) (Details) (USD $)	1 Months Ended	12 Months Ended
	Jul. 31, 2011	Dec. 31, 2011	Dec. 31, 2010
Derivative fixed interest rate	3.06%	3.06%
Swap contract notional amount fixed rate of interest and variable LIBOR	$ 200,000,000	$ 200,000,000	$ 189,000,000
Swap contract notional amount variable LIBOR based rate and fixed rate of interest			470,000,000
Proceeds from termination of interest rate derivatives	26,100,000
Terminated Interest Rate Derivative [Member]
Notional amount of fixed-for-variable interest rate derivative contracts	470,000,000
Termination date	July 2011
Roll Adjustment Swap Contracts [Member]
Asset value attributable to roll adjustment swaps		$ 181,000