Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Basic net income per share:
Net income	$ 20,798	$ 16,311	$ 22,367	$ 27,261	$ 29,907	$ 27,197	$ 25,134	$ 23,180	$ 86,737	$ 105,418	$ 76,411
Weighted average shares outstanding:
Common stock									138,577,000	135,279,000	129,922,000
Number of shares used in per share computations									138,577,000	135,279,000	129,922,000
Earnings per share	$ 0.15	$ 0.12	$ 0.16	$ 0.20	$ 0.22	$ 0.20	$ 0.19	$ 0.17	$ 0.63	$ 0.78	$ 0.59
Diluted net income per share:
Net income	$ 20,798	$ 16,311	$ 22,367	$ 27,261	$ 29,907	$ 27,197	$ 25,134	$ 23,180	$ 86,737	$ 105,418	$ 76,411
Weighted average shares outstanding:
Common stock									138,577,000	135,279,000	129,922,000
Stock options, awards and employee share purchase plans									4,434,000	5,986,000	8,142,000
Number of shares used in per share computations									143,011,000	141,265,000	138,064,000
Earnings per share	$ 0.14	$ 0.11	$ 0.16	$ 0.19	$ 0.21	$ 0.19	$ 0.18	$ 0.17	$ 0.61	$ 0.75	$ 0.55
Number of potential common shares excluded from the computation of dilutive earnings per share because the effect would have been anti-dilutive									3,500,000	1,100,000	1,200,000