Earnings per Share (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Calculation of basic and diluted earnings per share
Net income (loss)	$ 70,690,000	$ (38,336,000)	$ 163,939,000	$ 12,889,000
Basic weighted-average common shares outstanding	66,943	65,745	66,486	64,815
Add: dilutive effect of stock options, unvested RSU's, and contingent PSU's	1,310	0	1,483	1,870
Add: dilutive effect of 3.50% Senior Convertible Notes	0	0	0	658
Diluted weighted-average common shares outstanding	68,253	65,745	67,969	67,343
Basic net income (loss) per common share	$ 1.06	$ (0.58)	$ 2.47	$ 0.20
Diluted net income (loss) per common share	$ 1.04	$ (0.58)	$ 2.41	$ 0.19
Performance Shares [Member]
Earnings per share
Performance period to receive awards (in years)			3
Common stock awards vesting multiplier, low end of range			0
Common stock awards vesting multiplier, high end of range			2
Convertible Debt [Member]
Earnings per share
Interest rate on notes (as a percent)	3.50%		3.50%
Debt Instrument, Principal Amount Denominator for Conversion into Common Stock			$ 1,000
Common stock conversion price (in dollars per share)		$ 54.42		$ 54.42