Subsequent Events (Details) (USD $)	9 Months Ended	12 Months Ended	3 Months Ended							12 Months Ended		9 Months Ended				9 Months Ended				0 Months Ended									0 Months Ended									0 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Common Shares	Jun. 30, 2013 Common Shares	Mar. 31, 2013 Common Shares	Dec. 31, 2012 Common Shares	Sep. 30, 2012 Common Shares	Jun. 30, 2012 Common Shares	Mar. 31, 2012 Common Shares	Dec. 31, 2013 Common Shares	Dec. 31, 2012 Common Shares	Sep. 30, 2013 Unsecured Credit Facility	Dec. 31, 2012 Unsecured Credit Facility	Sep. 30, 2013 Unsecured Credit Facility Minimum	Sep. 30, 2013 Unsecured Credit Facility Maximum	Sep. 30, 2013 Bank of America Unsecured Term Loan	Dec. 31, 2012 Bank of America Unsecured Term Loan	Sep. 30, 2013 Bank of America Unsecured Term Loan Minimum	Sep. 30, 2013 Bank of America Unsecured Term Loan Maximum	Sep. 24, 2013 Subsequent event Dividends declared monthly Common Shares	Sep. 24, 2013 Subsequent event Dividends declared quarterly Common Shares	Oct. 07, 2013 Subsequent event Bank of America Unsecured Credit Facility and Unsecured Term Loan Original Terms	Oct. 07, 2013 Subsequent event Bank of America Unsecured Credit Facility and Unsecured Term Loan Eurodollar Rate Original Terms	Oct. 07, 2013 Subsequent event Bank of America Unsecured Credit Facility and Unsecured Term Loan Base Rate Original Terms	Oct. 07, 2013 Subsequent event Bank of America Unsecured Credit Facility and Unsecured Term Loan Minimum Eurodollar Rate Original Terms	Oct. 07, 2013 Subsequent event Bank of America Unsecured Credit Facility and Unsecured Term Loan Minimum Base Rate Original Terms	Oct. 07, 2013 Subsequent event Bank of America Unsecured Credit Facility and Unsecured Term Loan Maximum Eurodollar Rate Original Terms	Oct. 07, 2013 Subsequent event Bank of America Unsecured Credit Facility and Unsecured Term Loan Maximum Base Rate Original Terms	Oct. 07, 2013 Subsequent event Unsecured Credit Facility Amended Terms	Oct. 07, 2013 Subsequent event Unsecured Credit Facility LIBOR	Oct. 07, 2013 Subsequent event Unsecured Credit Facility Base Rate	Oct. 07, 2013 Subsequent event Unsecured Credit Facility Eurodollar Rate Amended Terms	Oct. 07, 2013 Subsequent event Unsecured Credit Facility Base Rate Amended Terms	Oct. 07, 2013 Subsequent event Unsecured Credit Facility Minimum Eurodollar Rate Amended Terms	Oct. 07, 2013 Subsequent event Unsecured Credit Facility Minimum Base Rate Amended Terms	Oct. 07, 2013 Subsequent event Unsecured Credit Facility Maximum Eurodollar Rate Amended Terms	Oct. 07, 2013 Subsequent event Unsecured Credit Facility Maximum Base Rate Amended Terms	Oct. 07, 2013 Subsequent event Bank of America Unsecured Term Loan LIBOR	Oct. 07, 2013 Subsequent event Bank of America Unsecured Term Loan Base Rate	Oct. 07, 2013 Subsequent event Bank of America Unsecured Term Loan Eurodollar Rate Amended Terms	Oct. 07, 2013 Subsequent event Bank of America Unsecured Term Loan Base Rate Amended Terms	Oct. 07, 2013 Subsequent event Bank of America Unsecured Term Loan Minimum Eurodollar Rate Amended Terms	Oct. 07, 2013 Subsequent event Bank of America Unsecured Term Loan Minimum Base Rate Amended Terms	Oct. 07, 2013 Subsequent event Bank of America Unsecured Term Loan Maximum Eurodollar Rate Amended Terms	Oct. 07, 2013 Subsequent event Bank of America Unsecured Term Loan Maximum Base Rate Amended Terms
Subsequent Events
Dividends on common stock declared (in dollars per share)			$ 0.30	$ 0.30	$ 0.30	$ 0.27	$ 0.27	$ 0.27	$ 0.26	$ 0.90	$ 1.07									$ 0.10	$ 0.30
Interest rate base	One-month LIBOR	One-month LIBOR										LIBOR				LIBOR							one month-LIBOR	Base Rate						one month LIBOR	Base Rate	one month-LIBOR	Base Rate					LIBOR	Base Rate	one month-LIBOR	Base Rate
Interest rate margin (as a percent)												1.65%	1.65%	1.65%	2.25%	1.65%	1.65%	1.65%	2.25%						1.65%	0.65%	2.25%	1.25%						1.45%	0.45%	2.05%	1.05%					1.40%	0.40%	2.00%	1.00%
Unused commitment fee when usage less than 50 % (as a percent)																						0.35%							0.25%
Unused commitment fee when usage more than or equal to 50 % (as a percent)																						0.25%							0.20%