New accounting standards (Details 8) - USD ($)	3 Months Ended			12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other accrued liabilities			$ 186,010,000	$ 221,059,000	$ 186,010,000
Deferred income taxes			347,271,000	430,085,000	347,271,000
Other			1,179,140,000	1,148,359,000	1,179,140,000
Retained earnings			1,040,748,000	1,163,602,000	1,040,748,000
Net unrealized gain (loss) on investments used to satisfy obligations under nonqualified benefit plans				(3,600,000)	9,300,000	$ 3,400,000
Income tax expense (benefit), continuing operations, adjustment of deferred tax (asset) liability			(39,500,000)	(42,000)	7,735,000	0
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
Restatement Adjustment | Accounting Standards Update 2017-07 | Other Nonoperating Income (Expense)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net periodic benefit cost					6,200,000	4,500,000
Net unrealized gain (loss) on investments used to satisfy obligations under nonqualified benefit plans					10,800,000	$ 4,700,000
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | 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)
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other accrued liabilities			186,010,000		186,010,000
Deferred income taxes			347,271,000		347,271,000
Other			1,179,140,000		1,179,140,000
Retained earnings			1,040,748,000		$ 1,040,748,000
Other comprehensive income (loss)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income tax expense (benefit), continuing operations, adjustment of deferred tax (asset) liability			$ 7,700,000
Other comprehensive income (loss) | Adjustments for New Accounting Principle, Early Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income tax expense (benefit), continuing operations, adjustment of deferred tax (asset) liability		$ 7,900,000
Subsequent event [Member] | Maximum [Member] | Liability | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New accounting pronouncement or change in accounting principle, effect of adoption, quantification	$ 125,000,000
Subsequent event [Member] | Minimum [Member] | Liability | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New accounting pronouncement or change in accounting principle, effect of adoption, quantification	$ 105,000,000