Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands											68 Months Ended
	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	Apr. 21, 2010	Dec. 31, 2015
Class of Stock [Line Items]
Shares Issued (in shares)											36,956,000
Gross Proceeds											$ 730,350
Net Proceeds											$ 693,700
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
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)	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)	$ 30	$ 21.85		$ 18.35		$ 16.35		$ 14.7	$ 16
Gross Proceeds	$ 105,000	$ 131,100		$ 59,600		$ 73,600		$ 51,400	$ 64,000
Net Proceeds	$ 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