Equity (Details 2) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jul. 01, 2015	Dec. 31, 2014
Equity
Preferred shares outstanding	2,000,000		2,000,000			2,000,000
Dividend Distributions
Declared	$ 37,883,000	$ 37,140,000	$ 37,883,000	$ 37,140,000
Paid			$ 37,883,000	$ 37,140,000
Dividends declared and paid per common share per month (in dollars per share)			$ 0.17	$ 0.17
Dividends declared and paid per common share (in dollars per share)	$ 0.510	$ 0.510	$ 1.020	$ 1.020
Accumulated other comprehensive income
Accumulated other comprehensive income	$ 65,000		$ 65,000			$ 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		800,000,000
Amount available under effective shelf registration statement	$ 775,100,000		$ 775,100,000
Series C Preferred Stock [Member]
Equity
Preferred shares outstanding	2,000,000		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		2,000,000
Dividend Distributions
Declared	$ 1,636,000	$ 1,636,000	$ 1,636,000	$ 1,636,000
Paid			$ 1,636,000	$ 1,636,000
Common Stock [Member]
Equity
Number of shares repurchased			4,609	200
Dividend Distributions
Declared	$ 36,247,000	$ 35,504,000	$ 36,247,000	$ 35,504,000
Paid			$ 36,247,000	$ 35,504,000