Significant Accounting Policies - Earnings per share (Details) - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Significant Accounting Policies
Net income (loss) available for stockholders	$ (87,111)	$ 175,402	$ 162,511
Less: Adjustments for losses attributable to participating securities	(334)	(1,839)	(2,102)
Net income (loss) used in calculating per share amounts	$ (87,445)	$ 173,563	$ 160,409
Weighted average shares outstanding (including participating securities)	214,730	215,198	217,194
Adjustment for participating securities	(2,509)	(2,453)	(2,891)
Shares used in calculating basic and diluted (loss) earnings per share	212,221	212,745	214,303