Stockholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended	3 Months Ended		1 Months Ended	3 Months Ended					1 Months Ended	3 Months Ended										1 Months Ended
	Feb. 29, 2012	Mar. 31, 2013	Mar. 31, 2012	Feb. 29, 2012 Series D Depositary Share [Member]	Mar. 31, 2013 Series D Depositary Share [Member]		Mar. 31, 2012 Series D Depositary Share [Member]		Dec. 31, 2012 Series D Depositary Share [Member]	Mar. 31, 2012 Series C Depositary Share [Member]	Mar. 31, 2013 Series C Depositary Share [Member]		Mar. 31, 2012 Series C Depositary Share [Member]		Mar. 31, 2013 DRIP and ATM Equity Program [Member]	Mar. 31, 2012 DRIP and ATM Equity Program [Member]	Mar. 31, 2013 ATM Equity Program 2012 [Member]		Mar. 31, 2012 ATM Equity Program 2012 [Member]		May 31, 2012 ATM Equity Program 2012 [Member] Common Stock [Member]	Mar. 31, 2013 ATM Equity Program 2013 [Member] Common Stock [Member]	Apr. 30, 2013 Subsequent Event [Member]
Class of Stock [Line Items]
Sale of Stock, Average Stock Price Per Share																	$ 32.56		$ 0.00
Stock Issued During Period, Value, New Issues																	$ 150,327		$ 0
Series C preferred stock dividends and Series D preferred stock dividends					4,762	[1]	0	[1]			0	[2]	1,979	[2]
Series C preferred stock and Series D preferred stock, dividends per share (in dollars per share)					$ 0.4141	[1]	$ 0.0000	[1]			$ 0.0000	[2]	$ 0.5378	[2]
Payment of common stock dividends		44,321	40,432
Common stock, dividends, per share, cash paid (in dollars per share)		$ 0.395	$ 0.385
Preferred stock, shares issued (in shares)				11,500,000	11,500,000				0
Preferred stock, dividend rate, percentage				6.625%						7.375%
Preferred stock, redemption price per share (in dollars per share)				$ 25.00						$ 25.00			$ 25.00
Proceeds from issuance of preferred stock		0	287,500	287,500
Payments of stock issuance costs		2,264	9,922	9,855													2,129	[3]	0	[3]
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			$ 25.0768229
Preferred stock redemption premium										3,098
Shares available for issuance under at market offering program (in shares)																					9,000,000	9,000,000
Issuance costs																						135
Stock issued during period																	4,616,542		0			0
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
Stock issued during period, dividend reinvestment plan (in shares)															424,995	1,428,069
Proceeds from issuance of common stock		$ 166,067	$ 37,550												$ 13,822	$ 37,666

[1] 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.

[3] Stock issuance costs consist primarily of underwriters' fees and commissions, and legal and accounting fees.