Stockholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended	9 Months Ended		1 Months Ended	3 Months Ended		9 Months Ended					1 Months Ended	3 Months Ended		9 Months Ended					9 Months Ended		1 Months Ended	9 Months Ended	1 Months Ended
	Feb. 29, 2012	Sep. 30, 2012	Sep. 30, 2011	Feb. 29, 2012 Series D Depositary Share [Member]	Sep. 30, 2012 Series D Depositary Share [Member]	Sep. 30, 2011 Series D Depositary Share [Member]	Sep. 30, 2012 Series D Depositary Share [Member]		Sep. 30, 2011 Series D Depositary Share [Member]		Dec. 31, 2011 Series D Depositary Share [Member]	Mar. 31, 2012 Series C Depositary Share [Member]	Sep. 30, 2012 Series C Depositary Share [Member]	Sep. 30, 2011 Series C Depositary Share [Member]	Sep. 30, 2012 Series C Depositary Share [Member]		Sep. 30, 2011 Series C Depositary Share [Member]		Dec. 31, 2011 Series C Depositary Share [Member]	Sep. 30, 2012 DRIP and ATM Equity Program [Member]	Sep. 30, 2011 DRIP and ATM Equity Program [Member]	May 31, 2012 ATM Equity Program [Member]	Sep. 30, 2012 ATM Equity Program [Member]	Oct. 31, 2012 Subsequent Event [Member]
Class of Stock [Line Items]
Series C preferred stock dividends and Series D preferred stock dividends					$ 4,762	$ 0	$ 10,688	[1]	$ 0	[1]			$ 0	$ 1,696	$ 1,979	[2]	$ 5,089	[2]
Series C preferred stock and Series D preferred stock, dividends per share (in dollars per share)							$ 0.9293	[1]	$ 0	[1]					$ 0.5378	[2]	$ 1.3828	[2]
Payment of common stock dividends		124,139	96,915
Common stock, dividends, per share, cash paid (in dollars per share)		$ 1.165	$ 1.145
Common stock, dividends, per share, declared (in dollars per share)																								$ 0.395
Common stock, shares reserved for future issuance, dividend reinvestment plan (in shares)	16,000,000
Preferred stock, shares issued (in shares)				11,500,000	11,500,000		11,500,000				0		0		0				3,680,000
Preferred stock, dividend rate, percentage				6.625%								7.375%
Preferred stock, redemption price per share (in dollars per share)				$ 25								$ 25
Proceeds from issuance of preferred stock		287,500	0	277,645
Payments of Stock Issuance Costs		11,200	10,224	9,855																		1,171
Stock redeemed or called during period (in shares)												3,680,000
Preferred stock, fixed annual dividend per share, depository shares (in dollars per share)												$ 25.0768229
Preferred stock redemption premium												3,098
Shares available for issuance under at market offering program (in shares)																						9,000,000
At market offering equity program, period in force																						3 years
Stock Issued During Period, Shares, New Issues (in shares)																							2,251,798
Sale of Stock, Average Stock Price Per Share (in dollars per share)																							$ 29.05
Stock Issued During Period, Value, New Issues																							65,420
Stock issued during period, dividend reinvestment plan (in shares)																				1,994,113	2,423,611
Proceeds from issuance of common stock		$ 119,259	$ 299,528																	$ 52,781	$ 59,707

[1] The Series D preferred stock dividends paid during the quarter ended June 30, 2012 include accumulated and unpaid dividends from the issuance date through the declaration date. The Series D preferred stock has no maturity date and will remain outstanding unless redeemed.

[2] The Series C preferred stock was redeemed in March 2012. The dividends paid during the quarter ended March 31, 2012 include accumulated and unpaid dividends through the redemption date.