Earnings per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Calculation of basic and diluted earnings per share
Net income	$ 208,938	$ 70,690	$ 334,325	$ 163,939
Basic weighted-average common shares outstanding	67,379	66,943	67,169	66,486
Add: dilutive effect of stock options, unvested RSU's, and contingent PSU's	1,051	1,310	1,089	1,483
Diluted weighted-average common shares outstanding	68,430	68,253	68,258	67,969
Basic net income per common share	$ 3.10	$ 1.06	$ 4.98	$ 2.47
Diluted net income per common share	$ 3.05	$ 1.04	$ 4.90	$ 2.41
Performance Shares [Member]
Earnings per share
Performance period to receive awards (in years)			3
Maximum [Member] | Performance Shares [Member]
Earnings per share
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage			200.00%
Minimum [Member] | Performance Shares [Member]
Earnings per share
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage			0.00%