Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended		9 Months Ended
		Oct. 31, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015		Sep. 30, 2014		Feb. 28, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Total allowable shares			375,000,000		375,000,000					375,000,000
Net proceeds					$ 127,485		$ 146,978
Total shares issued			135,485,173		135,485,173					132,010,104
Stock issuance costs					$ 2,206		2,192
Common Stock Dividends					$ 169,674		$ 151,274
Common Stock Per share					$ 1.275		$ 1.230
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	$ 14,285	[1]	$ 14,285	[1]
Preferred Stock Per share	[1]				$ 1.242188		$ 1.242188
Series E Preferred Stock
Class of Stock [Line Items]
Preferred Stock Dividends			$ 4,097	$ 4,097	$ 12,291	[1]	$ 12,291	[1]
Preferred Stock Per share	[1]				$ 1.068750		$ 1.068750
DRIP
Class of Stock [Line Items]
Total allowable shares									16,000,000
Shares of common stock					147,614		300,231
Net proceeds					$ 5,409		$ 10,171
2015 ATM
Class of Stock [Line Items]
Total allowable shares			10,000,000		10,000,000
Total shares issued			2,891,502		2,891,502
Shares of common stock					2,891,502
Average price per share (net)					$ 38.62
Net proceeds					$ 111,674
Stock issuance costs	[2]				$ 1,894
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		3,758,362
Average price per share (net)					$ 39.84		$ 35.91
Net proceeds					$ 8,525		$ 134,962
Stock issuance costs	[2]				$ 130		$ 2,153
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
Shares of common stock					0		0

[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.