Earnings Per Share Attributable to Stockholders (Computation of Basic and Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share Attributable to Stockholders [Abstract]
Basic, Net income attributable to stockholders													$ 75,027	$ 91,832	$ 136,214
Effect of dilutive securities: PSUs, Net income attributable to stockholders													(152)
Diluted, Net income attributable to stockholders													$ 74,875	$ 91,832	$ 136,214
Basic, WA number of shares													107,044,348	106,630,323	107,075,845
Effect of dilutive securities: PSUs, WA number of shares													353,880	91,997
Effect of dilutive securities: Stock options, WA number of shares													714,923	735,474	356,629
Diluted, WA number of shares													108,113,151	107,457,794	107,432,474
Basic, Per share amount	$ 0.34	$ 0.10	$ 0.16	$ 0.10	$ 0.26	$ (0.05)	$ 0.37	$ 0.28	$ 0.43	$ 0.19	$ 0.42	$ 0.22	$ 0.70	$ 0.86	$ 1.27
Effect of dilutive securities: Stock options, Per share amount													(0.01)	(0.01)
Diluted, Per share amount	$ 0.34	$ 0.09	$ 0.16	$ 0.10	$ 0.26	$ (0.05)	$ 0.37	$ 0.27	$ 0.43	$ 0.19	$ 0.42	$ 0.22	$ 0.69	$ 0.85	$ 1.27