Recently Adopted and Recently Issued Accounting Standards (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in Working Capital			$ 27,000
Cumulative Effect of New Accounting Principle in Period of Adoption			397
Income Tax Expense (Benefit)		$ 83,803	6,637	$ 27,678
Deferred Compensation, Share-based Payments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income Tax Expense (Benefit)	$ 400
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption		$ (1,000)	$ (709)