Pension Plans and Postretirement Benefits - Net Periodic Benefit Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 36	$ 36	$ 108	$ 109
Interest cost	59	53	178	158
Expected return on assets	(40)	(38)	(119)	(113)
Amortization of prior service cost	0	0	0	0
Amortization of unrecognized (gain) loss	(40)	(39)	(118)	(119)
Amortization of net transition obligation	0	0	0	0
Net periodic (benefit) cost	15	12	49	35
U.S. | Pension benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0	0	0	0
Interest cost	25	24	76	71
Expected return on assets	(40)	(38)	(119)	(113)
Amortization of prior service cost	0	0	0	0
Amortization of unrecognized (gain) loss	0	0	0	0
Amortization of net transition obligation	0	0	0	0
Net periodic (benefit) cost	(15)	(14)	(43)	(42)
U.S. | Other benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	36	36	108	109
Interest cost	34	29	102	87
Expected return on assets	0	0	0	0
Amortization of prior service cost	0	0	0	0
Amortization of unrecognized (gain) loss	(40)	(39)	(118)	(119)
Amortization of net transition obligation	0	0	0	0
Net periodic (benefit) cost	30	26	92	77
Canada
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	111	125	331	379
Interest cost	149	142	447	432
Expected return on assets	(202)	(199)	(605)	(602)
Amortization of prior service cost	13	13	38	39
Amortization of unrecognized (gain) loss	52	48	154	145
Amortization of net transition obligation	3	6	9	18
Exchange rate changes	30	(65)	(78)	80
Net periodic (benefit) cost	156	70	296	491
Canada | Pension benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	95	110	285	332
Interest cost	134	128	402	389
Expected return on assets	(202)	(199)	(605)	(602)
Amortization of prior service cost	13	13	38	39
Amortization of unrecognized (gain) loss	52	47	154	142
Amortization of net transition obligation	0	0	0	0
Exchange rate changes	47	(97)	(120)	121
Net periodic (benefit) cost	139	2	154	421
Canada | Other benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	16	15	46	47
Interest cost	15	14	45	43
Expected return on assets	0	0	0	0
Amortization of prior service cost	0	0	0	0
Amortization of unrecognized (gain) loss	0	1	0	3
Amortization of net transition obligation	3	6	9	18
Exchange rate changes	(17)	32	42	(41)
Net periodic (benefit) cost	$ 17	$ 68	$ 142	$ 70