Earnings Per Share (Details) (USD $) 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
Numerator:
Net income	$ 64,461	$ 92,006	$ 124,149	$ 160,273
Denominator:
Denominator for basic earnings per share — weighted-average shares outstanding	187,229	186,372	186,945	186,052
Effect of dilutive employee stock awards	1,257	2,907	1,805	3,030
Denominator for diluted earnings per share — weighted-average shares outstanding	188,486	189,279	188,750	189,082
Basic earnings per share (in dollars per share)	$ 0.34	$ 0.49	$ 0.66	$ 0.86
Diluted earnings per share (in dollars per share)	$ 0.34	$ 0.49	$ 0.66	$ 0.85
Anti-dilutive weighted-average shares	6,270	3,326	4,689	4,103