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Common stock Preferred stock and additional paid in capital-Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands | 1 Months Ended | 3 Months Ended | 6 Months Ended | 7 Months Ended | 10 Months Ended |  |
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May. 13, 2015 | Jan. 22, 2014 | Apr. 13, 2015 | Mar. 27, 2012 | Jun. 30, 2015 | Jun. 30, 2014 | Jul. 11, 2010 | Aug. 07, 2013 | Jul. 20, 2010 | Oct. 19, 2012 | Nov. 04, 2010 | Oct. 19, 2010 | Mar. 31, 2015 | Jul. 12, 2010
Preferred Stock Shares Issued |  |  |  |  |  | 6,000,000 |  |  |  |  |  |  |  |
Common Stock, Preferred Stock Issuance, value |  |  |  |  | $ 5,383 |  |  |  |  |  |  |  |  |
Common stock shares issued |  |  |  |  | 149,600 |  |  |  |  |  |  |  | 149,600 |
Common stock shares par value |  |  |  |  | $ 0.0001 |  |  |  |  |  |  |  | $ 0.0001 |
Dividends paid |  |  |  |  | $ (48,836) | $ (45,029) |  |  |  |  |  |  |  |
Issuance of four million Series C Cumulative Redeemable Perpetual Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividend per share |  | $ 2.125 |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock Shares Issued |  | 4,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock dividend rate |  | 8.50% |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, Redemption Terms |  | At any time after January 21, 2019, the Series C Preferred Stock may be redeemed, at Company's election at a price of $25 of liquidation preference per share. |  |  |  |  |  |  |  |  |  |  |  |
Public offering per share price |  | $ 25 |  |  |  |  |  |  |  |  |  |  |  |
Follow-on offering proceeds, net |  | $ 96,523 |  |  |  |  |  |  |  |  |  |  |  |
Follow-on offering completion [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock shares issued during period |  |  |  | 7,500,000 |  |  |  |  |  |  |  |  |  |
Common stock shares issued |  |  |  | 67,800,000 |  |  |  |  |  |  |  |  |  |
Common stock shares par value |  |  |  | $ 0.0001 |  |  |  |  |  |  |  |  |  |
Public offering per share price |  |  |  | $ 14.1 |  |  |  |  |  |  |  |  |  |
Follow-on offering proceeds, net |  |  |  | $ 100,584 |  |  |  |  |  |  |  |  |  |
October fifteen to January fourteen Series B Preferred Dividends paid [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends paid |  |  |  |  |  | $ 953 |  |  |  |  |  |  |  |
Dividends paid, per common share |  |  |  |  |  | $ 0.476563 |  |  |  |  |  |  |  |
January fifteen to April fourteen cy Series B Preferred Dividends paid [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends paid |  |  |  |  |  | $ 953 |  |  |  |  |  |  |  |
Dividends paid, per common share |  |  |  |  |  | $ 0.476563 |  |  |  |  |  |  |  |
January to April cy Series C Preferred Dividends paid [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends paid |  |  |  |  | $ 2,125 | $ 1,983 |  |  |  |  |  |  |  |
Dividends paid, per common share |  |  |  |  | $ 0.531250 | $ 0.495833 |  |  |  |  |  |  |  |
October fifteen to January fourteen Series B Preferred Dividends paid [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends paid |  |  |  |  | $ 953 |  |  |  |  |  |  |  |  |
Dividends paid, per common share |  |  |  |  | $ 0.476563 |  |  |  |  |  |  |  |  |
October to January cy Series C Preferred Dividends paid [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends paid |  |  |  |  | $ 2,125 |  |  |  |  |  |  |  |  |
Dividends paid, per common share |  |  |  |  | $ 0.531250 |  |  |  |  |  |  |  |  |
First Quarter Cy Dividends paid [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends paid |  |  |  |  | $ 21,736 | $ 20,944 |  |  |  |  |  |  |  |
Dividends paid, per common share |  |  |  |  | $ 0.28 | $ 0.28 |  |  |  |  |  |  |  |
Fourth Quarter Py Dividends paid [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends paid |  |  |  |  | $ 20,944 | $ 20,196 |  |  |  |  |  |  |  |
Dividends paid, per common share |  |  |  |  | $ 0.28 | $ 0.27 |  |  |  |  |  |  |  |
January to April cy Series B Preferred Dividends paid [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends paid |  |  | $ 953 |  |  |  |  |  |  |  |  |  |  |
Dividends paid, per common share |  |  | $ 0.476563 |  |  |  |  |  |  |  |  |  |  |
Companys authorized capital stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock authorization date |  |  |  |  |  |  |  |  | July 14, 2010 |  |  |  |  |
Common stock shares authorized |  |  |  |  |  |  | 2,000,000 |  |  |  |  |  |  |
Stock split date |  |  |  |  |  |  |  |  |  |  |  | August 27, 2010 |  |
Common stock shares issued during period |  |  |  |  |  |  | 1,000,000 |  | 24,000,000 |  |  | 22,000,000 |  |
Dividend per share |  |  |  |  |  |  |  |  |  |  |  | $ 0.88 |  |
Common Stock, Preferred Stock Issuance, value |  |  |  |  |  |  |  |  | $ 2,400 |  |  |  |  |
Common stock shares issued |  |  |  |  |  |  |  |  | 25,000,000 |  |  |  |  |
Common stock shares par value |  |  |  |  |  |  | $ 0.0001 |  |  |  |  |  |  |
Issuance of two million Series B Cumulative Redeemable Perpetual Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividend per share |  |  |  |  |  |  |  | $ 1.90625 |  |  |  |  |  |
Preferred Stock Shares Issued |  |  |  |  |  |  |  | 2,000,000 |  |  |  |  |  |
Preferred stock dividend rate |  |  |  |  |  |  |  | 7.625% |  |  |  |  |  |
Preferred Stock, Redemption Terms |  |  |  |  |  |  |  | At any time after August 6, 2018, the Series B Preferred Stock may be redeemed, at Company's election at a price of $25 of liquidation preference per share. |  |  |  |  |  |
Public offering per share price |  |  |  |  |  |  |  | $ 25 |  |  |  |  |  |
Follow-on offering proceeds, net |  |  |  |  |  |  |  | $ 48,042 |  |  |  |  |  |
Second follow-on offering completion [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock shares issued during period |  |  |  |  |  |  |  |  |  | 7,000,000 |  |  |  |
Common stock shares issued |  |  |  |  |  |  |  |  |  | 74,800,000 |  |  |  |
Common stock shares par value |  |  |  |  |  |  |  |  |  | $ 0.0001 |  |  |  |
Public offering per share price |  |  |  |  |  |  |  |  |  | $ 14 |  |  |  |
Follow-on offering proceeds, net |  |  |  |  |  |  |  |  |  | $ 93,547 |  |  |  |
Companys Ipo completion [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock shares issued during period |  |  |  |  |  |  |  |  |  |  | 13,300,000 |  |  |
Common stock shares issued |  |  |  |  |  |  |  |  |  |  | 60,300,000 |  |  |
Common stock shares par value |  |  |  |  |  |  |  |  |  |  | $ 0.0001 |  |  |
Public offering per share price |  |  |  |  |  |  |  |  |  |  | $ 12 |  |  |
Initial Public offering proceeds, net |  |  |  |  |  |  |  |  |  |  | $ 145,543 |  |  |
Authorized capital stock (amended articles incorporation) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock shares authorized |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,000,000,000
Common stock shares par value |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0001
Preferred Stock, Shares Authorized |  |  |  |  |  |  |  |  |  |  |  |  |  | 100,000,000
Preferred stock shares designated as Series A Participating Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |  | 10,000,000
Preferred stock shares par value |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0001
Series D Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividend per share | $ 2.1875 |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock Shares Issued | 4,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock dividend rate | 8.75% |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, Redemption Terms | At any time after May 13, 2020, the Series D Preferred Stock may be redeemed, at the Company’s election at a price of $25 of liquidation preference per share. |  |  |  |  |  |  |  |  |  |  |  |  |
Public offering per share price | $ 25 |  |  |  |  |  |  |  |  |  |  |  |  |
Follow-on offering proceeds, net | $ 96,616 |  |  |  |  |  |  |  |  |  |  |  |  |

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