Other comprehensive income (reclassification adjustments) (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification adjustments [Line Items]
Interest expense										$ (22,302)	$ (20,299)	$ (36,529)
Income tax provision										(111,004)	(109,318)	(97,387)
Net income		$ 54,228	$ 58,663	$ 58,389	$ 58,102	$ 59,709	$ 56,917	$ 56,063	$ 45,940	229,382	218,629	199,794
Total reclassifications, net of tax										(1,652)	(1,249)	(2,117)
Amortization of loss on interest rate locks [Member]
Reclassification adjustments [Line Items]
Total reclassifications, net of tax	[1]									0	0	(781)
Amortization of postretirement benefit plan items [Member]
Reclassification adjustments [Line Items]
Total reclassifications, net of tax										(1,652)	(1,249)	(1,336)
Amounts reclassified from accumulated other comprehensive loss [Member]
Reclassification adjustments [Line Items]
Total reclassifications, net of tax										(1,652)	(1,249)	(2,117)
Amounts reclassified from accumulated other comprehensive loss [Member] | Amortization of loss on interest rate locks [Member]
Reclassification adjustments [Line Items]
Interest expense	[2]									0	0	(1,282)
Income tax provision										0	0	501
Net income										0	0	(781)
Amounts reclassified from accumulated other comprehensive loss [Member] | Amortization of postretirement benefit plan items [Member]
Reclassification adjustments [Line Items]
Prior service credit	[3]									1,421	1,421	1,421
Net actuarial loss	[3]									(3,797)	(3,120)	(3,418)
Total amortization	[3]									(2,376)	(1,699)	(1,997)
Tax benefit	[3]									724	450	661
Amortization of postretirement benefit plan items, net of tax	[3]									$ (1,652)	$ (1,249)	$ (1,336)

[1] Relates to interest rate locks executed in 2004. Further information regarding these financial instruments can be found in Note 6.

[2] Relates to interest rate locks executed in 2004. Further information regarding these financial instruments can be found in Note 6.

[3] Amortization of postretirement benefit plan items is included in the computation of net periodic benefit income as presented in Note 12. Net periodic benefit income is included in cost of revenue and in SG&A expense in the consolidated statements of income, based on the composition of our workforce. A portion of net periodic benefit income is capitalized as a component of labor costs and is included in inventories and intangibles in our consolidated balance sheets.