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 Reconciliation [Abstract]
Net Income (Loss) Attributable to Parent	$ 31,376	$ 33,770	$ 57,196	$ 63,713
Undistributed Earnings (Loss) Allocated to Participating Securities, Basic	401	494	708	905
Net earnings available to common shareholders	$ 30,975	$ 33,276	$ 56,488	$ 62,808
Weighted-average outstanding shares of common stock - basic	47,547,118	48,055,783	47,505,406	48,011,250
Dilutive effect of equity incentive plans	235,814	209,515	239,367	221,095
Incremental Common Shares Attributable to Dilutive Effect of Contingently Issuable Shares	1,165,383	0	1,112,632	19,353
Weighted-average outstanding shares of common stock - diluted	48,948,315	48,265,298	48,857,405	48,251,698
Basic	$ 0.65	$ 0.69	$ 1.19	$ 1.31
Diluted	$ 0.63	$ 0.69	$ 1.16	$ 1.30
Share options excluded from the computation of diluted earnings per share	0	62,000	0	62,000