Earnings per Share (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings per share
Document Period End Date			Jun. 30, 2013
Calculation of basic and diluted earnings per share
Net income	$ 76,522,000	$ 24,889,000	$ 93,249,000	$ 51,225,000
Basic weighted-average common shares outstanding	66,295	64,585	66,254	64,345
Add: dilutive effect of stock options, unvested RSU's, and contingent PSU's	1,598	2,149	1,457	2,182
Add: dilutive effect of 3.50% Senior Convertible Notes	0	822	0	1,279
Diluted weighted-average common shares outstanding	67,893	67,556	67,711	67,806
Basic net income per common share	$ 1.15	$ 0.39	$ 1.41	$ 0.80
Diluted net income per common share	$ 1.13	$ 0.37	$ 1.38	$ 0.76
Performance Shares [Member]
Earnings per share
Performance period to receive awards (in years)			3
Common stock awards vesting multiplier, low end of range	0		0
Common stock awards vesting multiplier, high end of range	2		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