Stockholders' Equity (Details) (USD $)	1 Months Ended		6 Months Ended		1 Months Ended	3 Months Ended		6 Months Ended					1 Months Ended	3 Months Ended		6 Months Ended					6 Months Ended		1 Months Ended	3 Months Ended	6 Months Ended
	Jul. 31, 2012	Feb. 29, 2012	Jun. 30, 2012	Jun. 30, 2011	Feb. 29, 2012 Series D Depositary Share [Member]	Jun. 30, 2012 Series D Depositary Share [Member]	Jun. 30, 2011 Series D Depositary Share [Member]	Jun. 30, 2012 Series D Depositary Share [Member]		Jun. 30, 2011 Series D Depositary Share [Member]		Dec. 31, 2011 Series D Depositary Share [Member]	Mar. 31, 2012 Series C Depositary Share [Member]	Jun. 30, 2012 Series C Depositary Share [Member]	Jun. 30, 2011 Series C Depositary Share [Member]	Jun. 30, 2012 Series C Depositary Share [Member]		Jun. 30, 2011 Series C Depositary Share [Member]		Dec. 31, 2011 Series C Depositary Share [Member]	Jun. 30, 2012 DRIP and ATM Equity Program [Member]	Jun. 30, 2011 DRIP and ATM Equity Program [Member]	May 31, 2012 ATM Equity Program [Member]	Jun. 30, 2012 ATM Equity Program [Member] Y	Jun. 30, 2012 ATM Equity Program [Member] Y
Class of Stock [Line Items]
Series C preferred stock dividends and Series D preferred stock dividends						$ 5,926,000	$ 0	$ 5,926,000	[1]	$ 0	[1]			$ 0	$ 1,696,000	$ 1,979,000	[2]	$ 3,392,000	[2]
Series C preferred stock and Series D preferred stock, dividends per share								$ 0.5153	[1]	$ 0	[1]					$ 0.5378	[2]	$ 0.9218	[2]
Payment of common stock dividends			81,511,000	63,887,000
Common stock, dividends, per share, cash paid			$ 0.770	$ 0.760
Common stock, dividends, per share, declared	$ 0.395
Common stock, shares reserved for future issuance, dividend reinvestment plan		16,000,000
Preferred stock, shares issued					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					$ 25.00									$ 25.00		$ 25.00
Proceeds from issuance of preferred stock			287,500,000	0	277,645,000
Payments of Stock Issuance Costs			10,393,000	0	9,855,000																			253,000	253,000
Stock redeemed or called during period, shares													3,680,000
Preferred stock, fixed annual dividend per share, depository shares														$ 25.0768229		$ 25.0768229
Preferred stock redemption premium													3,098,000
Shares available for issuance under at market offering program																							9,000,000
At market offering equity program, period in force																								3	3
Stock Issued During Period, Shares, New Issues																								395,000	395,000
Sale of Stock, Average Stock Price Per Share																								$ 27.30	$ 27.30
Stock Issued During Period, Value, New Issues																								10,527,000	10,527,000
Stock issued during period, shares, dividend reinvestment plan																					1,881,807	2,284,335
Proceeds from issuance of common stock			$ 60,319,000	$ 56,391,000																	$ 49,476,000	$ 56,447,000

[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.