Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

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Name:	us-gaap_StockRepurchasedDuringPeriodShares
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

Shareholders' equity (Details) - USD ($) shares in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Oct. 24, 2018	May 04, 2016
Shareholders' equity [Line Items]
Common shares repurchased (in shares)	1,919
Payments for common shares repurchased	$ 120,000	$ 50,070
Share repurchase program, authorized amount				$ 300,000
Amount remaining under share repurchase authorization	$ 119,227
Subsequent Event [Member]
Shareholders' equity [Line Items]
Share repurchase program, authorized amount			$ 500,000