Earnings per Share (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
Calculation of basic and diluted earnings per share
Net income	$ 59,780	$ 76,522	$ 125,387	$ 93,249
Basic weighted-average common shares outstanding	67,069	66,295	67,063	66,254
Add: dilutive effect of stock options, unvested RSU's, and contingent PSU's	1,170	1,598	1,117	1,457
Diluted weighted-average common shares outstanding	68,239	67,893	68,180	67,711
Basic net income per common share	$ 0.89	$ 1.15	$ 1.87	$ 1.41
Diluted net income per common share	$ 0.88	$ 1.13	$ 1.84	$ 1.38
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