Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Gross deferred tax assets:
Pension, postretirement and other employee benefits	$ 49,669	$ 71,842
Tax credit carryforwards	77,897	17,967
Property, plant and equipment basis differences	97,242	0
Canadian pool of scientific research and experimentation deductions (SR&ED)	79,349	0
Environmental liabilities	36,791	54,351
Capitalized costs	6,347	10,894
U.S. federal and Canadian net operating losses	212,904	8,951
State net operating losses	2,946	3,102
Interest carryforwards	11,635	0
Other	1,868	0
Total gross deferred tax assets	576,648	167,107
Less: valuation allowance	(92,081)	(20,821)
Total deferred tax assets after valuation allowance	484,567	146,286
Gross deferred tax liabilities:
Property, plant and equipment basis differences	(95,754)	(92,287)
Intangible assets	(15,948)	0
Other	(2,626)	(2,753)
Total gross deferred tax liabilities	(114,328)	(95,040)
Net deferred tax asset	370,239	51,246
Deferred tax assets	402,846	51,246
Deferred tax liabilities	(32,607)	0
Deferred Assets (Liabilities)	$ 370,239	$ 51,246