Shareholders' equity - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year	$ (161,573)	$ (130,951)	$ (778,769)
Other comprehensive income (loss) before reclassifications	17,438	(32,920)	24,439
Spin-off publishing businesses		(2,642)	603,469
Amounts reclassified from AOCL	37,212	4,940	19,910
Balance at end of year	(106,923)	(161,573)	(130,951)
Retirement Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year	(124,978)	(114,133)	(1,169,882)
Other comprehensive income (loss) before reclassifications	12,496	(13,143)	23,094
Spin-off publishing businesses		(2,642)	1,012,745
Amounts reclassified from AOCL	5,445	4,940	19,910
Balance at end of year	(107,037)	(124,978)	(114,133)
Foreign Currency Translation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year	(28,560)	(20,129)	391,113
Other comprehensive income (loss) before reclassifications	6,649	(8,431)	(1,966)
Spin-off publishing businesses		0	(409,276)
Amounts reclassified from AOCL	22,025	0	0
Balance at end of year	114	(28,560)	(20,129)
Other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year	(8,035)	3,311	0
Other comprehensive income (loss) before reclassifications	(1,707)	(11,346)	3,311
Spin-off publishing businesses		0	0
Amounts reclassified from AOCL	9,742	0	0
Balance at end of year	$ 0	$ (8,035)	$ 3,311