Equity (Details 2) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Apr. 30, 2015
Equity
Preferred shares outstanding	2,000,000		2,000,000
Dividend Distributions
Declared	$ 18,934,000	$ 18,560,000
Paid	18,934,000	18,560,000
Dividends declared and paid per common share per month (in dollars per share)	$ 0.17	$ 0.17
Accumulated other comprehensive income
Accumulated other comprehensive income	73,000		82,000
Subsequent Event [Member] | Dividend Declared [Member]
Dividend Distributions
Cash dividend declared per common share per month (in dollars per share)	$ 0.17
Shelf Registration [Member]
Equity
Maximum offering capacity under shelf registration statement	800,000,000
Amount available under effective shelf registration statement	775,100,000
Series C Preferred Stock [Member]
Equity
Preferred shares outstanding	2,000,000
Dividend Rate (as a percent)	8.50%
Number of shares of common stock to be issued upon conversion	2,000,000
Conversion price per share	$ 19.25
Total shares reserved for issuance of common stock related to the conversion of preferred stock	2,000,000
Dividend Distributions
Declared	818,000	818,000
Paid	818,000	818,000
Common Stock [Member]
Equity
Number of shares repurchased	4,609	200
Dividend Distributions
Declared	18,116,000	17,742,000
Paid	$ 18,116,000	$ 17,742,000
Common Stock [Member] | Subsequent Event [Member]
Dividend Distributions
Cash dividend declared per common share per month (in dollars per share)				$ 0.17