Stockholders' Equity (Details 2) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended			12 Months Ended
	Jan. 31, 2011	Dec. 31, 2010	Jan. 05, 2004	Dec. 31, 2011 Nonvested restricted stock awards Y	Dec. 31, 2010 Nonvested restricted stock awards	Dec. 31, 2009 Nonvested restricted stock awards	Dec. 31, 2011 Nonvested restricted stock units Y	Dec. 31, 2010 Nonvested restricted stock units	Dec. 31, 2009 Nonvested restricted stock units
Nonvested restricted stock awards and units
Outstanding, beginning of period (in shares)				22,309	28,910	321,935	190,488	184,454	176,112
Awarded (in shares)				18,748	22,309	30,385	115,003	101,812	121,065
Vested (in shares)				(22,309)	(28,910)	(319,547)	(90,853)	(84,615)	(73,465)
Forfeited (in shares)						(3,863)	(8,300)	(11,163)	(39,258)
Outstanding, ending of period (in shares)				18,748	22,309	28,910	206,338	190,488	184,454
Weighted average exercise price of nonvested restricted stock award and units
Outstanding, beginning of period (in dollars per share)				$ 39.23	$ 30.27	$ 42.92	$ 38.43	$ 34.99	$ 38.72
Awarded (in dollars per share)				$ 52.11	$ 39.23	$ 30.36	$ 49.14	$ 42.75	$ 32.91
Vested (in dollars per share)				$ 39.23	$ 30.27	$ 42.97	$ 37.50	$ 36.20	$ 39.16
Forfeited (in dollars per share)						$ 36.66	$ 42.94	$ 37.97	$ 37.60
Outstanding, ending of period (in dollars per share)				$ 52.11	$ 39.23	$ 30.27	$ 44.63	$ 38.43	$ 34.99
Unrecognized compensation expense				$ 0.4			$ 3.8
Unrecognized compensation expense, period of recognition (in years)				0.38			1.89
Warrants issued to purchase common stock (in shares)			570,825
Fair value per warrant			$ 9.44
Purchase price of common stock (in dollars per share)			$ 30.46
Warrants outstanding at the end of the period (in shares)		44,561
Exercise of stock warrants (in shares)	31,362
Warrants forfeited (in shares)	13,199