Shareholders' Equity - Schedule of Common Share Offerings (Details) - USD ($) $ / shares in Units, $ in Thousands		2 Months Ended	35 Months Ended	39 Months Ended	51 Months Ended		54 Months Ended		59 Months Ended	60 Months Ended	83 Months Ended	92 Months Ended
	Apr. 21, 2010	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Nov. 09, 2017	Jan. 27, 2015	Sep. 24, 2014	Oct. 11, 2013	Sep. 30, 2013	Jun. 28, 2013	Jun. 18, 2013	Jan. 31, 2013	Jan. 14, 2013	Feb. 08, 2011
Class of Stock [Line Items]
Shares Issued (in shares)												41,956,000
Gross Proceeds												$ 839,850
Net Proceeds												$ 802,400
Initial public offering
Class of Stock [Line Items]
Shares Issued (in shares)												8,625,000
Price per Share (in dollars per share)	$ 20
Gross Proceeds												$ 172,500
Net Proceeds												$ 158,700
Private placement offering
Class of Stock [Line Items]
Shares Issued (in shares)	500,000											500,000
Price per Share (in dollars per share)	$ 20
Gross Proceeds												$ 10,000
Net Proceeds												$ 10,000
Follow-on common share offering
Class of Stock [Line Items]
Shares Issued (in shares)		5,000,000	3,500,000	6,000,000		3,250,000	4,500,000			3,500,000	4,000,000
Price per Share (in dollars per share)													$ 21.90	$ 30	$ 21.85		$ 18.35		$ 16.35		$ 14.7	$ 16
Gross Proceeds		$ 109,500	$ 105,000	$ 131,100		$ 59,600	$ 73,600			$ 51,400	$ 64,000
Net Proceeds		$ 108,700	$ 103,300	$ 125,600		$ 56,700	$ 70,000			$ 48,400	$ 60,300
Over-allotment option
Class of Stock [Line Items]
Shares Issued (in shares)			525,000	900,000	487,500			475,823	92,677		600,000
Price per Share (in dollars per share)														$ 30	$ 21.85	$ 18.35		$ 16.35		$ 14.7		$ 16
Gross Proceeds			$ 15,750	$ 19,700	$ 8,900			$ 7,800	$ 1,400		$ 9,600
Net Proceeds			$ 15,500	$ 18,900	$ 8,500			$ 7,400	$ 1,300		$ 9,100