Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended		6 Months Ended
		Jul. 31, 2015	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015		Jun. 30, 2014		Dec. 31, 2014	Feb. 29, 2012
Class of Stock [Line Items]
Net proceeds					$ 88,694		$ 76,098
Total allowable shares			375,000,000		375,000,000				375,000,000
Total shares issued			134,434,132		134,434,132				132,010,104
Stock issuance costs					$ 1,623		1,104
Common Stock Dividends					$ 111,400		$ 98,958
Common Stock Per share					$ 0.840		$ 0.810
Subsequent Event
Class of Stock [Line Items]
Common stock, dividends, per share, declared (in dollars per share)		$ 0.435
Series D Preferred Stock
Class of Stock [Line Items]
Preferred Stock Dividends			$ 4,762	$ 4,762	$ 9,523	[1]	$ 9,523	[1]
Preferred Stock Per share	[1]				$ 0.828125		$ 0.828125
Series E Preferred Stock
Class of Stock [Line Items]
Preferred Stock Dividends			$ 4,096	$ 4,096	$ 8,194	[1]	$ 8,194	[1]
Preferred Stock Per share	[1]				$ 0.712500		$ 0.712500
DRIP
Class of Stock [Line Items]
Shares of common stock					82,271		290,399
Net proceeds					$ 3,019		$ 9,814
Total allowable shares										16,000,000
2015 ATM
Class of Stock [Line Items]
Total allowable shares			10,000,000		10,000,000
Total shares issued			1,910,873		1,910,873
Shares of common stock					1,910,873
Average price per share (net)					$ 39.70
Net proceeds					$ 75,857
Stock issuance costs	[2]				$ 1,312
2013 ATM
Class of Stock [Line Items]
Total allowable shares			9,000,000		9,000,000
Total shares issued			6,252,812		6,252,812
Shares of common stock					214,000		1,861,206
Average price per share (net)					$ 39.84		$ 35.21
Net proceeds					$ 8,525		$ 65,526
Stock issuance costs	[2]				$ 130		$ 1,065
2012 ATM
Class of Stock [Line Items]
Total allowable shares			9,000,000		9,000,000
Total shares issued			8,958,840		8,958,840

[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.