Stockholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended	3 Months Ended				1 Months Ended
	Nov. 30, 2014	Mar. 31, 2015		Mar. 31, 2014		Apr. 30, 2015	Dec. 31, 2014	Feb. 29, 2012
Class of Stock [Line Items]
Underwriters over allotment of shares	712,500
Shares Issued, price per share	$ 38.16
Stock issuance costs	$ 8,488
Net proceeds	199,961	49,370		21,144
Total allowable shares		375,000,000					375,000,000
Total shares issued		133,446,857					132,010,104
Shares of common stock	5,462,500
Stock issuance costs		991		304
Common Stock Dividends		55,314		49,273
Common Stock Per share		$ 0.420		$ 0.405
Subsequent Event
Class of Stock [Line Items]
Common stock, dividends, per share, declared (in dollars per share)						$ 0.42
Series D Preferred Stock
Class of Stock [Line Items]
Preferred Stock Dividends		4,762	[1]	4,762	[1]
Preferred Stock Per share		$ 0.414063	[1]	$ 0.414063	[1]
Series E Preferred Stock
Class of Stock [Line Items]
Preferred Stock Dividends		4,097	[1]	4,097	[1]
Preferred Stock Per share		$ 0.356250	[1]	$ 0.356250	[1]
DRIP
Class of Stock [Line Items]
Shares of common stock		31,854		184,503
Net proceeds		1,142		6,107
Total allowable shares								16,000,000
ATM Equity Program 2015
Class of Stock [Line Items]
Total allowable shares		10,000,000
Total shares issued		977,973
Shares of common stock		977,973
Average price per share (net)		$ 39.91
Net proceeds		39,031
Stock issuance costs		712	[2]
ATM Equity Program 2013
Class of Stock [Line Items]
Total allowable shares		9,000,000
Total shares issued							6,252,812
Shares of common stock		214,000		432,000
Average price per share (net)		$ 39.84		$ 34.91
Net proceeds		8,525		15,080
Stock issuance costs		$ 130	[2]	$ 265	[2]
ATM Equity Program 2012
Class of Stock [Line Items]
Total allowable shares		9,000,000
Total shares issued							8,958,840

[1] The Series D and E preferred stock have no maturity date and will remain outstanding unless redeemed.

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