Earnings per share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Earnings per share - basic:
Net income	$ 58,663	$ 56,917	$ 175,154	$ 158,920
Income allocated to participating securities	(491)	(386)	(1,445)	(1,054)
Income available to common shareholders	$ 58,172	$ 56,531	$ 173,709	$ 157,866
Weighted-average shares outstanding	48,493	49,396	48,634	49,592
Earnings per share - basic	$ 1.20	$ 1.14	$ 3.57	$ 3.18
Earnings per share - diluted:
Net income	$ 58,663	$ 56,917	$ 175,154	$ 158,920
Income allocated to participating securities	(487)	(384)	(1,436)	(1,049)
Re-measurement of share-based awards classified as liabilities	(64)	(114)	230	(67)
Income available to common shareholders	$ 58,112	$ 56,419	$ 173,948	$ 157,804
Weighted-average shares outstanding	48,493	49,396	48,634	49,592
Dilutive impact of potential common shares	455	366	427	391
Weighted-average shares and potential common shares outstanding	48,948	49,762	49,061	49,983
Earnings per share - diluted	$ 1.19	$ 1.13	$ 3.55	$ 3.16
Antidilutive options excluded from calculation	223	255	223	255