Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended
		Apr. 30, 2016	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Feb. 28, 2015
Class of Stock [Line Items]
Total allowable shares (in shares)			375,000,000		375,000,000
Net proceeds			$ 89,135	$ 49,370
Total shares issued (in shares)			143,235,896		141,007,725
Stock issuance costs			$ 1,380	991
Common stock dividends			$ 61,151	$ 55,314
Common stock per share (in dollars per share)			$ 0.435	$ 0.42
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	[1]		$ 4,762	$ 4,762
Preferred stock per share (in dollars per share)	[1]		$ 0.414063	$ 0.414063
Series E Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends	[1]		$ 4,097	$ 4,097
Preferred stock per share (in dollars per share)	[1]		$ 0.356250	$ 0.356250
DRIP
Class of Stock [Line Items]
Total allowable shares (in shares)						16,000,000
Shares of common stock (in shares)			134,150	31,854
Net proceeds			$ 5,892	$ 1,142
2016 ATM
Class of Stock [Line Items]
Total allowable shares (in shares)			12,000,000
Total shares issued (in shares)			372,328
Shares of common stock (in shares)			1,865,260
Average price per share (net) (in dollars per share)			$ 44.00
Net proceeds			$ 82,076
Stock issuance costs			$ 1,429
2015 ATM
Class of Stock [Line Items]
Total allowable shares (in shares)			10,000,000
Total shares issued (in shares)			9,852,465
Shares of common stock (in shares)			1,191,973
Average price per share (net) (in dollars per share)			$ 39.90
Net proceeds			$ 47,556
Stock issuance costs			$ 842
2013 ATM
Class of Stock [Line Items]
Total allowable shares (in shares)			9,000,000
Total shares issued (in shares)			6,252,812
Shares of common stock (in shares)			0

[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.