Shareholders' equity - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 995,041	$ 995,041	$ 2,553,005	$ 2,456,744
Other comprehensive (loss) income before reclassifications		(14,182)	17,438	(32,920)
Amounts reclassified from AOCL		9,439	37,212	4,940
Other comprehensive (loss) income, net of tax		(4,743)	54,209	(35,487)
Spin-off publishing businesses				(2,642)
Reclassification of stranded tax effects to retained earnings	(24,800)	(24,845)
Ending Balance		1,340,924	995,041	2,553,005
Retirement Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(107,037)	(107,037)	(124,978)	(114,133)
Other comprehensive (loss) income before reclassifications		(14,450)	12,496	(13,143)
Amounts reclassified from AOCL		9,439	5,445	4,940
Other comprehensive (loss) income, net of tax		(5,011)
Spin-off publishing businesses				(2,642)
Reclassification of stranded tax effects to retained earnings		(24,845)
Ending Balance		(136,893)	(107,037)	(124,978)
Foreign Currency Translation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	114	114	(28,560)	(20,129)
Other comprehensive (loss) income before reclassifications		268	6,649	(8,431)
Amounts reclassified from AOCL		0	22,025	0
Other comprehensive (loss) income, net of tax		268
Spin-off publishing businesses				0
Reclassification of stranded tax effects to retained earnings		0
Ending Balance		382	114	(28,560)
Other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	0	0	(8,035)	3,311
Other comprehensive (loss) income before reclassifications			(1,707)	(11,346)
Amounts reclassified from AOCL			9,742	0
Spin-off publishing businesses				0
Ending Balance			0	(8,035)
AOCI Including Portion Attributable to Noncontrolling Interest
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ (106,923)	(106,923)	(161,573)	(130,951)
Ending Balance		$ (136,511)	$ (106,923)	$ (161,573)

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Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount after tax, before reclassification adjustments of other comprehensive income (loss).

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Amount after tax and reclassification adjustments of other comprehensive income (loss).

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Amount of stockholders' equity (deficit), net of receivables from officers, directors, owners, and affiliates of the entity, attributable to both the parent and noncontrolling interests. Amount excludes temporary equity. Alternate caption for the concept is permanent equity.

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