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	Apr. 30, 2014 Senior Notes Due 2019 [Member]
Numerator:
Net income	$ 53,024	$ 64,461	$ 108,963	$ 124,149
Denominator:
Denominator for basic earnings per share - weighted-average shares outstanding	169,973	187,229	176,946	186,945
Effect of dilutive employee stock awards	918	1,257	1,300	1,805
Denominator for diluted earnings per share - weighted-average shares outstanding	170,891	188,486	178,246	188,750
Basic earnings per share (in dollars per share)	$ 0.31	$ 0.34	$ 0.62	$ 0.66
Diluted earnings per share (in dollars per share)	$ 0.31	$ 0.34	$ 0.61	$ 0.66
Anti-dilutive weighted-average shares from stock awards	4,365	6,270	4,478	4,689
Convertible debt, conversion price (in dollars per share)					$ 90