Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended
	Apr. 30, 2018	Mar. 31, 2018	Mar. 31, 2017	Feb. 28, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Total allowable shares (in shares)		375,000,000			375,000,000
Stock issuance costs		$ 174	$ 798
Net proceeds		$ 1,554	49,011
Total shares issued (in shares)		153,848,013			153,577,028
Common stock dividends		$ 72,733	$ 66,781
Common stock per share (in dollars per share)		$ 0.475	$ 0.455
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	$ 3,598
Per depository share (in dollars per share)		$ 0	$ 0.312847
Series E Preferred Stock
Class of Stock [Line Items]
Dividends		$ 4,097	$ 4,097
Per depository share (in dollars per share)		$ 0.356250	$ 0.356250
Series F Preferred Stock
Class of Stock [Line Items]
Dividends		$ 4,485	$ 4,485
Per depository share (in dollars per share)		$ 0.325	$ 0.325000
DRIP
Class of Stock [Line Items]
Total allowable shares (in shares)				10,000,000
Shares of common stock (in shares)		44,659	16,320
Net proceeds		$ 1,491	$ 714
2018 ATM
Class of Stock [Line Items]
Total allowable shares (in shares)		12,000,000
Total shares issued (in shares)		0
2016 ATM
Class of Stock [Line Items]
Total allowable shares (in shares)		12,000,000
Total shares issued (in shares)		10,044,656
ATM Equity Programs
Class of Stock [Line Items]
Stock issuance costs			$ 743
Shares of common stock (in shares)			1,087,355
Average price per share (net) (in dollars per share)			$ 43.92
Net proceeds			$ 47,757