Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Feb. 28, 2015
Class of Stock [Line Items]
Total allowable shares (in shares)		375,000,000		375,000,000		375,000,000
Net proceeds				$ 174,499	$ 277,076
Total shares issued (in shares)		151,627,386		151,627,386		147,149,945
Stock issuance costs				$ 2,618	4,266
Common stock dividends		$ 70,670	$ 66,676	$ 205,110	$ 190,277
Common stock per share (in dollars per share)		$ 0.475	$ 0.455	$ 1.385	$ 1.325
Subsequent Event
Class of Stock [Line Items]
Common stock, dividends, per share, declared (in dollars per share)	$ 0.475
Series D Preferred Stock
Class of Stock [Line Items]
Dividends		$ 0	$ 4,762	$ 3,598	$ 14,285
Per depository share (in dollars per share)		$ 0	$ 0.414063	$ 0.3128472	$ 1.242188
Series E Preferred Stock
Class of Stock [Line Items]
Dividends		$ 4,097	$ 4,097	$ 12,291	$ 12,291
Per depository share (in dollars per share)		$ 0.356250	$ 0.356250	$ 1.068750	$ 1.068750
Series F Preferred Stock
Class of Stock [Line Items]
Dividends		$ 4,485	$ 0	$ 13,455	$ 0
Per depository share (in dollars per share)		$ 0.325	$ 0	$ 0.975	$ 0
DRIP
Class of Stock [Line Items]
Total allowable shares (in shares)							16,000,000
Shares of common stock (in shares)				135,721	164,640
Net proceeds				$ 5,469	$ 7,376
2016 ATM
Class of Stock [Line Items]
Total allowable shares (in shares)		12,000,000		12,000,000
Total shares issued (in shares)		8,192,542		8,192,542
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
ATM Equity Programs
Class of Stock [Line Items]
Shares of common stock (in shares)				3,969,252	5,716,222
Average price per share (net) (in dollars per share)				$ 42.00	$ 46.48
Net proceeds				$ 166,698	$ 265,696
Stock issuance costs				$ 2,563	$ 4,266