Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended		6 Months Ended
		Jul. 31, 2016	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015	Feb. 28, 2015
Class of Stock [Line Items]
Total allowable shares (in shares)			375,000,000		375,000,000		375,000,000
Net proceeds					$ 219,448	$ 88,694
Total shares issued (in shares)			146,002,140		146,002,140		141,007,725
Stock issuance costs					$ 3,379	1,623
Common stock dividends			$ 62,451	$ 56,086	$ 123,602	$ 111,400
Common stock per share (in dollars per share)			$ 0.435	$ 0.42	$ 0.87	$ 0.840
Subsequent Event
Class of Stock [Line Items]
Common stock, dividends, per share, declared (in dollars per share)		$ 0.455
Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends	[1]		$ 4,762	$ 4,762	$ 9,523	$ 9,523
Preferred stock per share (in dollars per share)	[1]		$ 0.414063	$ 0.414063	$ 0.828125	$ 0.828125
Series E Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends	[1]		$ 4,096	$ 4,096	$ 8,194	$ 8,194
Preferred stock per share (in dollars per share)	[1]		$ 0.356250	$ 0.356250	$ 0.712500	$ 0.712500
DRIP
Class of Stock [Line Items]
Total allowable shares (in shares)								16,000,000
Shares of common stock (in shares)					149,543	82,271
Net proceeds					$ 6,609	$ 3,019
2016 ATM
Class of Stock [Line Items]
Total allowable shares (in shares)			12,000,000		12,000,000
Total shares issued (in shares)			3,120,490		3,120,490
Shares of common stock (in shares)					4,613,422
Average price per share (net) (in dollars per share)					$ 45.46
Net proceeds					$ 209,722
Stock issuance costs					$ 3,379
2015 ATM
Class of Stock [Line Items]
Total allowable shares (in shares)			10,000,000		10,000,000
Total shares issued (in shares)			9,852,465		9,852,465
Shares of common stock (in shares)						2,124,873
Average price per share (net) (in dollars per share)						$ 39.71
Net proceeds						$ 84,382
Stock issuance costs						$ 1,442
2013 ATM
Class of Stock [Line Items]
Total allowable shares (in shares)			9,000,000		9,000,000
Total shares issued (in shares)			6,252,812		6,252,812

[1] The Series D and E preferred stock have no maturity date and will remain outstanding unless redeemed. The earliest redemption date for the Series D and Series E preferred stock is February 2017 and May 2018, respectively.