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
Numerator
Net income	$ 67,000	$ 61,103	$ 169,555	$ 133,725
Denominator
Weighted-average shares outstanding, basic	72,755	72,632	72,727	72,571
Effect of dilutive stock awards (in shares)	548	371	557	363
Adjusted weighted-average shares outstanding, diluted	73,303	73,003	73,284	72,934
Net Income per Share
Basic earnings per common share (in dollars per share)	$ 0.92	$ 0.84	$ 2.33	$ 1.84
Diluted earnings per common share (in dollars per share)	$ 0.91	$ 0.84	$ 2.31	$ 1.83
Anti-dilutive weighted-average shares	0	1	38	1