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				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, 2013	Sep. 30, 2012	Feb. 29, 2012 Series D Depositary Share [Member]	Sep. 30, 2013 Series D Depositary Share [Member]	Sep. 30, 2012 Series D Depositary Share [Member]	Sep. 30, 2013 Series D Depositary Share [Member]		Sep. 30, 2012 Series D Depositary Share [Member]		Dec. 31, 2012 Series D Depositary Share [Member]	Mar. 31, 2012 Series C Depositary Share [Member]	Sep. 30, 2013 Series C Depositary Share [Member]	Sep. 30, 2012 Series C Depositary Share [Member]	Sep. 30, 2013 Series C Depositary Share [Member]		Sep. 30, 2012 Series C Depositary Share [Member]		May 31, 2013 Series E Depositary Share [Member]	Sep. 30, 2013 Series E Depositary Share [Member]	Sep. 30, 2012 Series E Depositary Share [Member]	Sep. 30, 2013 Series E Depositary Share [Member]		Sep. 30, 2012 Series E Depositary Share [Member]		Dec. 31, 2012 Series E Depositary Share [Member]	Sep. 30, 2013 DRIP and ATM Equity Program [Member]	Sep. 30, 2012 DRIP and ATM Equity Program [Member]	Sep. 30, 2013 ATM Equity Program 2012 [Member]		Sep. 30, 2012 ATM Equity Program 2012 [Member]		May 31, 2012 ATM Equity Program 2012 [Member] Common Stock [Member]	Sep. 30, 2013 ATM Equity Program 2013 [Member]		Mar. 31, 2013 ATM Equity Program 2013 [Member] Common Stock [Member]	Oct. 31, 2013 Subsequent Event [Member]
Class of Stock [Line Items]
Dividends					$ 4,762	$ 4,762	$ 14,285	[1]	$ 10,688	[1]			$ 0	$ 0	$ 0	[2]	$ 1,979	[2]		$ 4,780	$ 0	$ 4,780	[1],[3]	$ 0	[1],[3]
Per share							$ 1.2422	[1]	$ 0.9293	[1]					$ 0.0000	[2]	$ 0.5378	[2]				$ 0.4156	[1],[3]	$ 0.0000	[1],[3]
Dividends		139,890	124,139
Per share		$ 1.195	$ 1.165
Shares of common stock																													4,676,542		2,251,798			2,280,450
Average price per share (net)																													$ 32.60		$ 28.97			$ 37.82
Net proceeds																													152,435		65,245			86,237
Common stock, dividends, per share, declared (in dollars per share)																																					$ 0.405
Preferred stock, shares issued (in shares)				11,500,000	11,500,000		11,500,000				0								11,500,000	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 issuance costs		13,455	11,200	9,855															9,856										2,161	[4]	1,171	[4]		1,585	[4]
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, shares reserved for future issuance, dividend reinvestment plan (in shares)	16,000,000
Shares of common stock																											634,146	1,994,113
Net proceeds		$ 263,366	$ 119,259																								$ 21,159	$ 52,781

[1] The Series E preferred stock dividends paid during the quarter ended September 30, 2013 include accumulated and unpaid dividends from the issuance date through the declaration date.

[2] The Series C preferred stock was redeemed in March 2012. The dividends paid during the nine months ended September 30, 2012 include accumulated and unpaid dividends through the redemption date.

[3] The Series D and E preferred stock have no maturity date and will remain outstanding unless redeemed.

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