Significant Accounting Policies - Summary of reconciliation of weighted average shares outstanding along with earnings per share attributable to restricted shares of common stock (Details 1) - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Significant Accounting Policies
Net income (loss) available for stockholders	$ 162,511	$ (141,246)	$ (99,561)
Less: Adjustments for earnings/ loss attributable to participating securities	(2,102)	(147)	(240)
Net income (loss) used in calculating earnings (loss) per share	$ 160,409	$ (141,393)	$ (99,801)
Weighted average shares outstanding (including participating securities)	217,194	217,509	213,632
Adjustment for participating securities	(2,891)	(3,282)	(3,359)
Shares used in calculating basic and diluted earnings (loss) per share	214,303	214,227	210,273