Tax Treatment of Distributions Tax Treatment of Distributions (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Class of Stock [Line Items]
Dividends declared per common share	$ 1.4	$ 1.4	$ 1.4
Common Stock [Member]
Class of Stock [Line Items]
Dividends declared per common share	$ 1.400	$ 1.400	$ 1.400
Less: Dividends declared in the current year and paid in the following year	$ (0.350)	$ (0.350)	$ (0.350)
Add: Dividends declared in the prior year and paid in the current year	$ 0.350	$ 0.350	$ 0.350
Dividends paid per common share	$ 1.400	$ 1.400	$ 1.400