Pension Plans and Postretirement Benefits - Components of Defined Benefit Pension Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
U.S. Pension Plans
Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year	$ 205,405	$ 207,025
Interest cost	8,119	8,464	$ 12,958
Actuarial (gains) losses	6,644	(544)
Benefits paid	(8,911)	(9,540)
Projected benefit obligation at end of year	211,257	205,405	207,025
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	127,514	117,719
Actual return on plan assets	22,192	2,947
Employer contributions	12,417	19,100
Administrative expenses	(2,406)	(2,712)
Benefits paid	(8,911)	(9,540)
Fair value of plan assets at end of year	150,806	127,514	117,719
Funded status at end of year	(60,451)	(77,891)
German Pension Plan
Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year	24,166	23,545
Interest cost	426	529	523
Actuarial (gains) losses	91	(1,157)
Currency adjustment	2,976	39
Benefits paid	(1,233)	(1,104)
Projected benefit obligation at end of year	26,244	24,166	$ 23,545
Change in Fair Value of Plan Assets [Roll Forward]
Benefits paid	$ (1,233)	$ (1,104)