Pension and Other Postretirement Benefits - Change in Benefit Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Change in benefit obligation:
Benefit obligation, beginning of year	$ (256,481)	$ (275,205)
Service cost	(4,978)	(4,981)	$ (5,505)
Interest cost	(7,671)	(8,909)	(9,116)
Participant contributions	(91)	(106)
Actuarial gain (loss)	(3,291)	(16,250)
Divestiture	794	0
Settlements	49	29,256
Curtailments	0	227
Foreign currency exchange rate changes	(14,299)	10,723
Benefits paid	13,943	8,764
Benefit obligation, end of year	(272,025)	(256,481)	(275,205)
Other Benefits [Member]
Change in benefit obligation:
Benefit obligation, beginning of year	(32,038)	(32,313)
Service cost	(49)	(46)	(52)
Interest cost	(1,139)	(1,259)	(1,301)
Participant contributions	(7)	(7)
Actuarial gain (loss)	3,370	578
Divestiture	0	0
Settlements	0	0
Curtailments	0	0
Foreign currency exchange rate changes	(2,022)	(580)
Benefits paid	1,552	1,589
Benefit obligation, end of year	$ (30,333)	$ (32,038)	$ (32,313)