Stockholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended	3 Months Ended							1 Months Ended	3 Months Ended					3 Months Ended					1 Months Ended	3 Months Ended				1 Months Ended
	Feb. 29, 2012	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014 Series D Depositary Share		Mar. 31, 2013 Series D Depositary Share		Dec. 31, 2013 Series D Depositary Share	May 31, 2013 Series E Depositary Share	Mar. 31, 2014 Series E Depositary Share		Mar. 31, 2013 Series E Depositary Share		Dec. 31, 2013 Series E Depositary Share	Mar. 31, 2014 DRIP and ATM Equity Program	Mar. 31, 2013 DRIP and ATM Equity Program	Mar. 31, 2014 ATM Equity Program 2012	Mar. 31, 2013 ATM Equity Program 2012		May 31, 2012 ATM Equity Program 2012 Common Stock	Mar. 31, 2014 ATM Equity Program 2013		Mar. 31, 2013 ATM Equity Program 2013		Mar. 31, 2013 ATM Equity Program 2013 Common Stock	Apr. 30, 2014 Subsequent Event
Class of Stock [Line Items]
Dividends				$ 4,762	[1]	$ 4,762	[1]			$ 4,097	[1]	$ 0	[1]
Per share				$ 0.414063	[1]	$ 0.414063	[1]			$ 0.356250	[1]	$ 0.000000	[1]
Dividends		49,273	44,321
Per share		$ 0.405	$ 0.395
Shares of common stock																	0	4,616,542			432,000		0
Average price per share (net)																		$ 32.56			$ 34.91
Net proceeds																		150,327			15,080
Stock issuance costs		304	2,264						9,856									2,129	[2]		265	[2]	135	[2]
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	11,500,000				11,500,000
Preferred stock, dividend rate, percentage									5.70%
Preferred stock, redemption price per share (in dollars per share)									$ 25.00
Proceeds from issuance of Series E preferred stock									287,500
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															184,503	424,995
Net proceeds		$ 21,144	$ 166,067												$ 6,107	$ 13,822

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

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