Equity (Details 2) (USD $)	6 Months Ended		12 Months Ended						6 Months Ended		1 Months Ended												6 Months Ended					6 Months Ended	12 Months Ended	6 Months Ended
	Jun. 30, 2014 item	Jun. 30, 2013	Dec. 31, 2012 item	Dec. 31, 2013	Sep. 30, 2014 Subsequent Event [Member] Dividend Declared [Member]	Aug. 31, 2014 Subsequent Event [Member] Dividend Declared [Member]	Jul. 31, 2014 Subsequent Event [Member] Dividend Declared [Member]	Jul. 19, 2013 Shelf Registration [Member]	Jun. 30, 2014 Series C Preferred Stock [Member]	Jun. 30, 2013 Series C Preferred Stock [Member]	Jun. 30, 2014 Common Stock [Member]	May 31, 2014 Common Stock [Member]	Apr. 30, 2014 Common Stock [Member]	Mar. 31, 2014 Common Stock [Member]	Feb. 28, 2014 Common Stock [Member]	Jan. 31, 2014 Common Stock [Member]	Jun. 30, 2013 Common Stock [Member]	May 31, 2013 Common Stock [Member]	Apr. 30, 2013 Common Stock [Member]	Mar. 31, 2013 Common Stock [Member]	Feb. 28, 2013 Common Stock [Member]	Jan. 31, 2013 Common Stock [Member]	Jun. 30, 2014 Common Stock [Member]	Jun. 30, 2013 Common Stock [Member]	Sep. 30, 2014 Common Stock [Member] Subsequent Event [Member] Dividend Declared [Member]	Aug. 31, 2014 Common Stock [Member] Subsequent Event [Member] Dividend Declared [Member]	Jul. 31, 2014 Common Stock [Member] Subsequent Event [Member] Dividend Declared [Member]	Jun. 30, 2013 Common Stock [Member] Equity Distribution Agreement [Member]	Dec. 31, 2013 Common Stock [Member] Equity Distribution Agreement [Member]	Jun. 30, 2013 Common Stock [Member] Underwritten Public Offering [Member]
Equity
Shares outstanding	2,000,000			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
Number of shares repurchased																							200	600
Maximum aggregate offering price of shares authorized for issuance under terminated agreement																												$ 85,686,000
Shares common stock sold																												126,742		4,025,000
Net proceeds																												4,895,000		171,406,000
Reclassification of accumulated costs to additional paid in capital																													662,000
Maximum offering capacity under shelf registration statement								800,000,000
Non-controlling Interests
Number of limited partners	0
Number of limited partnerships			1
Limited partnership, conversion basis			1
Payment of accrued distributions to former limited partners		7,000
Dividend Distributions
Declared	37,140,000	31,385,000							1,636,000	1,636,000	35,504,000						29,749,000						35,504,000	29,749,000
Paid	37,140,000	31,385,000							1,636,000	1,636,000													35,504,000	29,749,000
Cash dividend declared per common share per month (in dollars per share)											$ 0.17	$ 0.17	$ 0.17	$ 0.17	$ 0.17	$ 0.17	$ 0.155	$ 0.155	$ 0.155	$ 0.155	$ 0.155	$ 0.155
Cash dividend declared per common share per month (in dollars per share)					$ 0.17	$ 0.17	$ 0.17																		$ 0.17	$ 0.17	$ 0.17
Sale price of common stock (in dollars per share)																														$ 44.50
Fees and costs																														7,707,000
Accumulated other comprehensive income
Accumulated other comprehensive income	$ 99,000			$ 117,000