Tax Treatment of Distributions (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Tax Disclosure [Abstract]
Ordinary income	$ 73,295	$ 65,712	$ 47,041
Return of capital	82,870	94,952	69,686
Income distributed as dividends	$ 156,165	$ 160,664	$ 116,727
Ordinary income	46.90%	40.90%	40.30%
Return of capital	53.10%	59.10%	59.70%
Income distributed as dividends	100.00%	100.00%	100.00%