Stockholders' Equity (Details 3) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Restricted stock activity
Restricted shares outstanding, beginning of period	$ 31.89	$ 22.87	$ 24.15
Restricted shares outstanding, beginning of period, shares	834,703	734,286	703,856
Restricted shares granted	$ 31.12	$ 40.28	$ 25.41
Restricted shares granted, shares	543,800	499,050	388,650
Restricted shares canceled	$ 33.95	$ 32.29	$ 24.45
Restricted shares canceled, shares	(107,182)	(49,661)	(46,029)
Restricted shares vested	$ 26.62	$ 24.84	$ 28.75
Restricted shares vested, shares	(375,157)	(348,972)	(312,191)
Restricted shares outstanding, end of period	$ 33.38	$ 31.89	$ 22.87
Restricted shares outstanding, end of period, shares	896,164	834,703	734,286