Net Income Per Share (Tables)

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Schedule of Earnings Per Share, Basic and Diluted [Table Text Block]

The following table presents the computation of net income per share on a basic and diluted basis for the years ended December 31 (shares in thousands):

	2017		2016		2015
Net income attributable to shareholders	$	401,962	$	522,750	$	497,726
Weighted-average shares outstanding - basic	88,681		90,960		94,608
Net effect of various dilutive stock-based compensation awards	1,085		1,073		1,078
Weighted-average shares outstanding - diluted	89,766		92,033		95,686
Net income per share:
Basic	$	4.53	$	5.75	$	5.26
Diluted (a)	$	4.48	$	5.68	$	5.20

(a) Stock-based compensation awards for the issuance of 380 shares, 766 shares, and 658 shares for the years ended December 31, 2017, 2016, and 2015, respectively, were excluded from the computation of net income per share on a diluted basis as their effect was anti-dilutive.