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Shareholders' equity (Tables)

12 Months Ended

Dec. 31, 2017

Stockholders' Equity Note [Abstract]

Schedule of Common and Preferred Stock Activity

The Common and Preferred Stock Activity table below contains additional information about preferred stock conversions during 2017, 2016 and 2015:

Common and Preferred Stock Activity:	Preferred stock			Common stock
(in shares)	Series A Convertible			Treasury			Outstanding
Balance as of December 31, 2014	78,141			4,786,521			89,064,582
Repurchase of common stock	—			2,400,000			(2,400,000	)
Conversion of convertible preferred stock	(1,602	)		—			160,162
Issuance for share-based compensation plans	—			—			213,306
Beginning balance as of December 31, 2015	76,539			7,186,521			87,038,050
Repurchase of common stock	—			—			—
Conversion of convertible preferred stock	(914	)		—			91,362
Issuance for share-based compensation plans	—			—			121,485
Beginning balance as of December 31, 2016	75,625			7,186,521			87,250,897
Repurchase of common stock	—			—			—
Conversion of convertible preferred stock	(1,261	)		—			126,098
Issuance for share-based compensation plans	—			—			167,782
Ending balance as of December 31, 2017	74,364			7,186,521			87,544,777

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