Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Gross deferred tax assets:
U.S. federal and Canadian net operating losses	$ 205,563	$ 203,330
Canadian pool of scientific research and experimentation deductions (SR&ED)	87,315	87,253
Tax credit carryforwards	74,503	80,182
Property, plant and equipment basis differences	66,653	57,073
Pension, postretirement and other employee benefits	52,126	56,687
Environmental liabilities	39,118	36,583
Deferred interest deductions	15,537	5,820
Capitalized costs	2,979	5,275
State net operating losses	3,249	2,942
Other	12,818	9,738
Total gross deferred tax assets	559,861	544,883
Less: valuation allowance	(94,660)	(85,938)
Total deferred tax assets after valuation allowance	465,201	458,945
Gross deferred tax liabilities:
Property, plant and equipment basis differences	(90,290)	(92,857)
Intangible assets	(12,284)	(15,579)
Other	(2,961)	(3,054)
Total gross deferred tax liabilities	(105,535)	(111,490)
Net deferred tax asset	359,666	347,455
Deferred tax assets	384,513	375,471
Deferred tax liabilities	(24,847)	(28,016)
Deferred tax assets (liabilities)	$ 359,666	$ 347,455