New accounting standards ASU 2014-09 (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other accrued liabilities	$ 221,620,000		$ 186,010,000	$ 184,580,000
Deferred income taxes	405,761,000		347,271,000	652,413,000
Other	1,154,366,000		1,179,140,000	889,494,000
Retained earnings	1,124,830,000		1,040,748,000	$ 964,275,000
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New accounting pronouncement or change in accounting principle, effect of adoption, quantification	$ 0
Other accrued liabilities		$ 186,913,000
Deferred income taxes		346,939,000
Other		1,179,539,000
Retained earnings		1,039,778,000
Calculated under Revenue Guidance in Effect before Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other accrued liabilities			186,010,000
Deferred income taxes			347,271,000
Other			1,179,140,000
Retained earnings			$ 1,040,748,000
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other accrued liabilities		903,000
Deferred income taxes		(332,000)
Other		399,000
Retained earnings		$ (970,000)