Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended		6 Months Ended
		Jul. 31, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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					$ 74,503	$ 219,448
Total shares issued (in shares)			149,206,493		149,206,493		147,149,945
Stock issuance costs					$ 1,238	3,379
Common stock dividends			$ 67,660	$ 62,451	$ 134,440	$ 123,602
Common stock per share (in dollars per share)			$ 0.455	$ 0.435	$ 0.91	$ 0.870
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]
Preferred stock dividends			$ 0	$ 4,762	$ 3,598	$ 9,523
Preferred stock per share (in dollars per share)			$ 0	$ 0.414063	$ 0.312847	$ 0.828125
Series E Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends			$ 4,096	$ 4,096	$ 8,194	$ 8,194
Preferred stock per share (in dollars per share)			$ 0.356250	$ 0.356250	$ 0.712500	$ 0.712500
Series F Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends			$ 4,485	$ 0	$ 8,970	$ 0
Preferred stock per share (in dollars per share)			$ 0.32500	$ 0	$ 0.65	$ 0
DRIP
Class of Stock [Line Items]
Total allowable shares (in shares)								16,000,000
Shares of common stock (in shares)					34,466	149,543
Net proceeds					$ 1,413	$ 6,609
2016 ATM
Class of Stock [Line Items]
Total allowable shares (in shares)			12,000,000		12,000,000
Total shares issued (in shares)			5,876,445		5,876,445
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)					1,653,155	4,613,422
Average price per share (net) (in dollars per share)					$ 43.64	$ 45.46
Net proceeds					$ 72,139	$ 209,722
Stock issuance costs	[1]				$ 1,183	$ 3,379

[1] Stock issuance costs consist primarily of underwriters' fees and commissions, and legal and accounting fees.