Earnings per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Calculation of basic and diluted earnings per share
Net income	$ 65,607	$ 16,727
Basic weighted-average common shares outstanding	67,056	66,211
Add: dilutive effect of stock options, unvested RSU's, and contingent PSU's	1,070	1,310
Diluted weighted-average common shares outstanding	68,126	67,521
Basic net income per common share	$ 0.98	$ 0.25
Diluted net income per common share	$ 0.96	$ 0.25
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