Equity (Details) (USD $)	9 Months Ended			9 Months Ended		12 Months Ended	9 Months Ended		12 Months Ended	9 Months Ended	0 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended				0 Months Ended	3 Months Ended	0 Months Ended		9 Months Ended		0 Months Ended
	Sep. 30, 2013	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Stock Options	Sep. 30, 2014 Series A preferred shares	Dec. 31, 2013 Series A preferred shares	Sep. 30, 2014 Series A preferred shares Change in control provision Maximum	Sep. 30, 2014 Series B preferred shares	Dec. 31, 2013 Series B preferred shares	Sep. 30, 2014 Series B preferred shares Change in control provision Maximum	Mar. 20, 2013 Series C preferred shares	Sep. 30, 2014 Series C preferred shares	Dec. 31, 2013 Series C preferred shares	Sep. 30, 2014 Series C preferred shares Change in control provision Maximum	Sep. 30, 2014 Common Stock	Sep. 30, 2013 Common Stock	Sep. 30, 2014 Common Stock Stock Options	May 28, 2014 Common Stock Executive officers and employees	Sep. 30, 2014 Common Stock Executive officers and employees	Mar. 01, 2013 Common Stock Executive officers	Jun. 13, 2013 Common Stock Director	Sep. 30, 2014 Common Stock Director	Sep. 30, 2013 Common Stock Director	Sep. 19, 2013 Common Stock Public offering	Jan. 14, 2013 Common Stock Public offering
Equity
Shares of common stock issued to redeem common units															198,292	2,466,404
New issued stock (in shares)											3,400,000													17,250,000	17,250,000
Stock issued to officers and directors (in shares)																		278,916		292,090	29,228	32,317
Stock forfeited (in shares)																			1,756
Shares issued															319,590	325,758	4,253
Shares issued in lieu of cash for director fees																						5,860	4,440
Exercise price (in dollars per share)				$ 9.75
Proceeds from equity offerings, net of offering costs	$ 389,389,000										$ 81,700,000													$ 152,000,000
Underwriting discount and offering-related expenses											$ 3,300,000													$ 6,500,000	$ 7,200,000
Preferred stock, shares authorized		100,000,000	100,000,000		2,000,000	2,000,000		3,000,000	3,000,000			3,400,000	3,400,000
Preferred stock, par value (in dollars per share)		$ 0.01	$ 0.01
Preferred stock, dividend rate (as a percent)					9.25%	9.25%		7.875%	7.875%			7.125%	7.125%
Preferred stock, liquidation preference (in dollars per share)					$ 25			$ 25				$ 25
Ratio for conversion							5.92417			5.6497				5.1440
Annual dividend rate per share (in dollars per share)					$ 2.3125			$ 1.96875				$ 1.78125