Reconciliation of Earnings Per Share (EPS) - Earnings Per Share Reconciliation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net Income (Loss), Basic EPS	$ 118,915	$ 33,403
Basic Average Common Shares Outstanding (in shares)	97,321	97,140
Earnings Per Share, Basic (in dollars per share)	$ 1.22	$ 0.34
Effect of dilutive securities
Net Income (Loss), Diluted EPS	$ 118,915	$ 33,403
Diluted Average Common Shares Outstanding (in shares)	97,818	97,607
Diluted EPS (in dollars per share)	$ 1.22	$ 0.34
Stock options
Effect of dilutive securities
Effect of dilutive securities (in shares)	48	31
Non-vested restricted stock
Effect of dilutive securities
Effect of dilutive securities (in shares)	282	262
Performance share awards
Effect of dilutive securities
Effect of dilutive securities (in shares)	167	174