Stockholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended	6 Months Ended		1 Months Ended	3 Months Ended		6 Months Ended					1 Months Ended	3 Months Ended		6 Months Ended				1 Months Ended	6 Months Ended			6 Months Ended						1 Months Ended	6 Months Ended				1 Months Ended
	Feb. 29, 2012	Jun. 30, 2013	Jun. 30, 2012	Feb. 29, 2012 Series D Depositary Share [Member]	Jun. 30, 2013 Series D Depositary Share [Member]	Jun. 30, 2012 Series D Depositary Share [Member]	Jun. 30, 2013 Series D Depositary Share [Member]		Jun. 30, 2012 Series D Depositary Share [Member]		Dec. 31, 2012 Series D Depositary Share [Member]	Mar. 31, 2012 Series C Depositary Share [Member]	Jun. 30, 2013 Series C Depositary Share [Member]	Jun. 30, 2012 Series C Depositary Share [Member]	Jun. 30, 2013 Series C Depositary Share [Member]		Jun. 30, 2012 Series C Depositary Share [Member]		May 31, 2013 Series E Depositary Share [Member]	Jun. 30, 2013 Series E Depositary Share [Member]	Jun. 30, 2012 Series E Depositary Share [Member]	Dec. 31, 2012 Series E Depositary Share [Member]	Jun. 30, 2013 DRIP and ATM Equity Program [Member]	Jun. 30, 2012 DRIP and ATM Equity Program [Member]	Jun. 30, 2013 ATM Equity Program 2012 [Member]		Jun. 30, 2012 ATM Equity Program 2012 [Member]		May 31, 2012 ATM Equity Program 2012 [Member] Common Stock [Member]	Jun. 30, 2013 ATM Equity Program 2013 [Member]		Jun. 30, 2012 ATM Equity Program 2013 [Member]		Mar. 31, 2013 ATM Equity Program 2013 [Member] Common Stock [Member]	Jul. 31, 2013 Subsequent Event [Member]
Class of Stock [Line Items]
Dividends					$ 4,762	$ 5,926	$ 9,523	[1]	$ 5,926	[1]			$ 0	$ 0	$ 0	[2]	$ 1,979	[2]
Per share							$ 0.8281	[1]	$ 0.5153	[1]					$ 0.0000	[2]	$ 0.5378	[2]
Dividends		90,943	81,511
Per share		$ 0.790	$ 0.770
Shares of common stock																									4,676,542		394,901			2,280,450		0
Average price per share (net)																									$ 32.60		$ 26.66			$ 37.83		$ 0.00
Net proceeds																									152,435		10,527			86,271		0
Stock issuance costs		13,416	10,393	9,855															9,856						2,161	[3]	253	[3]		1,551	[3]	0	[3]
Dividends Payable																			$ 0.415625
Preferred stock, shares issued (in shares)				11,500,000	11,500,000		11,500,000				0								11,500,000	11,500,000		0
Preferred stock, dividend rate, percentage				6.625%								7.375%							5.70%
Preferred stock, redemption price per share (in dollars per share)				$ 25.00								$ 25.00							$ 25.00
Proceeds from issuance of Series E preferred stock				287,500			0		287,500										287,500	287,500	0
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					9,000,000
Common stock, dividends, per share, declared (in dollars per share)																																			$ 0.405
Common stock, shares reserved for future issuance, dividend reinvestment plan (in shares)	16,000,000
Shares of common stock																							475,866	1,881,807
Net proceeds		$ 258,085	$ 60,319																				$ 15,881	$ 49,476

[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 six months ended June 30, 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.