Income Taxes - Tax Status of Dividends (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Income Tax Contingency [Line Items]
Dividend per share	$ 1.85	$ 1.85	$ 2.11
Allocation of Dividends, Ordinary Income	33.00%	40.00%	54.00%
Allocation of Dividends, Capital Gain	1.00%	2.00%	14.00%
Allocation of Dividends, Return of Capital	66.00%	58.00%	32.00%