Earnings Per Share Attributable to Stockholders (Computation of Basic and Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Earnings Per Share Attributable to Stockholders [Abstract]
Basic, Net income attributable to stockholders	$ 17,635	$ 39,710	$ 28,012	$ 69,116
Effect of dilutive securities: PSUs, Net income attributable to stockholders	(545)		(176)
Diluted, Net income attributable to stockholders	$ 17,090	$ 39,710	$ 27,836	$ 69,116
Basic, WA number of shares	107,004,902	106,245,307	106,928,672	106,581,294
Effect of dilutive securities: PSUs, WA number of shares	534,849		399,203
Effect of dilutive securities: Stock options, WA number of shares	698,909	734,503	686,353	488,116
Diluted, WA number of shares	108,238,660	106,979,810	108,014,228	107,069,410
Basic, Per share amount	$ 0.16	$ 0.37	$ 0.26	$ 0.65
Diluted, Per share amount	$ 0.16	$ 0.37	$ 0.26	$ 0.65