Stockholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	9 Months Ended			3 Months Ended		9 Months Ended					1 Months Ended	3 Months Ended		9 Months Ended					9 Months Ended				9 Months Ended					9 Months Ended			1 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Sep. 30, 2014 Series D Depositary Share	Sep. 30, 2013 Series D Depositary Share	Sep. 30, 2014 Series D Depositary Share		Sep. 30, 2013 Series D Depositary Share		Dec. 31, 2013 Series D Depositary Share	May 31, 2013 Series E Depositary Share	Sep. 30, 2014 Series E Depositary Share	Sep. 30, 2013 Series E Depositary Share	Sep. 30, 2014 Series E Depositary Share		Sep. 30, 2013 Series E Depositary Share		Dec. 31, 2013 Series E Depositary Share	Sep. 30, 2014 ATM Equity Program 2012	Sep. 30, 2013 ATM Equity Program 2012		May 31, 2012 ATM Equity Program 2012	Sep. 30, 2014 ATM Equity Program 2013		Sep. 30, 2013 ATM Equity Program 2013		Mar. 31, 2013 ATM Equity Program 2013	Sep. 30, 2014 DRIP	Sep. 30, 2013 DRIP	Feb. 29, 2012 DRIP	Oct. 31, 2014 Subsequent Event
Class of Stock [Line Items]
Dividends				$ 4,762	$ 4,762	$ 14,285	[1]	$ 14,285	[1]			$ 4,097	$ 4,780	$ 12,291	[1]	$ 4,780	[1]
Per share						$ 1.242188	[1]	$ 1.242188	[1]					$ 1.06875	[1]	$ 0.415625	[1]
Dividends	151,274	139,890
Per share	$ 1.230	$ 1.195
Shares of common stock																			0	4,676,542			3,758,362		2,280,450
Average price per share (net)																				$ 32.60			$ 35.91		$ 37.82
Net proceeds																				152,435			134,962		86,237
Stock issuance costs	2,192	13,455									9,856									2,161	[2]		2,153	[2]	1,585	[2]
Common stock, dividends, per share, declared (in dollars per share)																															$ 0.42
Preferred stock, shares issued (in shares)				11,500,000		11,500,000				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	0	287,500									287,500
Common stock, shares authorized	375,000,000		375,000,000																			9,000,000					9,000,000			16,000,000
Common stock, shares issued	126,421,631		121,991,677																8,958,840				6,038,812
Shares of common stock																												300,231	634,146
Net proceeds	$ 146,978	$ 263,366																										$ 10,171	$ 21,159

[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.