Employee Benefit Plans (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Safety Incentive Plan	$ 4,300,000	$ 1,500,000	$ 200,000
Percentage of employee compensation contribution	6.00%
Percentage of vesting contribution	25.00%
Vesting period of employees	5 years
The matching contribution of employees	9,000,000	8,400,000	8,100,000
Employee Compensation (Additional Textual) [Abstract]
Aggregate amounts deferred under Officer Deferred Compensation Plan	1,698,000	2,807,000
Accrued Incentive Compensation expense	6,000,000	2,600,000	1,600,000
Earnings before tax upon achieving a specified earnings threshold	8.25%
Safety Incentive Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Safety Incentive Plan	$ 700,000	$ 600,000	$ 600,000