Common Stock (Details 2) (USD $)	12 Months Ended	3 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Restricted Stock [Member]	Sep. 30, 2011 Restricted Stock [Member]	Jun. 30, 2011 Restricted Stock [Member]	Mar. 31, 2011 Restricted Stock [Member]
Nonvested restricted shares
Nonvested Restricted Shares, Beginning Balance				413,539	415,861	425,521	558,198
Nonvested Restricted Shares, Granted				109	1,042	1,381	141,036
Restricted Shares, Vested				(5,735)	(3,206)	(3,851)	(268,891)
Nonvested Restricted Shares, Forfeited				(162)	(158)	(7,190)	(4,822)
Nonvested Restricted Shares, Ending Balance				407,751	413,539	415,861	425,521
Weighted Average Grant Date Fair Value
Nonvested, Weighted Average Grant Date Fair Value, Beginning of Period				$ 22.74	$ 22.62	$ 22.51	$ 11.58
Nonvested, Granted, Weighted Average Grant Date Fair Value	$ 45.38	$ 19.60	$ 2.02	$ 24.97	$ 44.02	$ 43.51	$ 45.42
Vested Weighted Average Grant Date Fair Value				$ 29.92	$ 15.41	$ 15.47	$ 12.00
Nonvested, Forfeited, Weighted Average Grant Date Fair Value				$ 1.87	$ 1.86	$ 23.82	$ 13.74
Nonvested, Weighted Average Grant Date Fair Value, End of Period				$ 22.64	$ 22.74	$ 22.62	$ 22.51

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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The weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect during the reporting period as a result of the occurrence of a terminating event specified in the contractual agreement of the plan.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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