Reconciliation of Earnings Per Share (EPS) - Earnings Per Share Reconciliation (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Earnings Per Share Reconciliation
Net Income, Basic EPS	$ (7,953)	$ 83,067	$ 45,363	$ 139,759
Basic Shares Outstanding (in shares)	72,851	72,167	72,737	72,155
Earnings Per Share, Basic (in dollars per share)	$ (0.11)	$ 1.15	$ 0.62	$ 1.94
Effect of dilutive securities
Net Income, Diluted EPS	$ (7,953)	$ 83,067	$ 45,363	$ 139,759
Diluted Shares Outstanding (in shares)	72,851	72,419	73,031	72,329
Earnings Per Share, Diluted (dollars per share)	$ (0.11)	$ 1.15	$ 0.62	$ 1.93
Stock options
Effect of dilutive securities
Incremental common shares attributable to share-based payment arrangements	0	225	165	164
Non-vested restricted stock
Effect of dilutive securities
Incremental common shares attributable to share-based payment arrangements	0	16	48	10
Performance share awards
Effect of dilutive securities
Incremental common shares attributable to share-based payment arrangements	0	0	81	0