Net Earnings Per Share Net Earnings Per Share (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net Income (loss) Available to Common Stockholders, Basic	$ 73,151	$ 63,529	$ 183,909	$ 157,513
Weighted Average Number of Shares Outstanding, Basic	138,190	137,849	138,146	137,030
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	2,137	1,570	2,467	1,823
Weighted Average Number of Shares Outstanding, Diluted	140,327	139,419	140,613	138,853