Employee Benefit Plans (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined contribution plan
Employer contribution to the plan	$ 1,900,000	$ 1,900,000	$ 1,800,000
Pension Plan
Employee benefit plans
Minimum age for eligibility under the Pension Plan	21
Minimum period of service for eligibility under the Pension Plan	5 years
Normal retirement benefit (as a percent)	2.00%
Number of highest consecutive years' average earnings used in computing retirement benefit	5 years
Maximum number of years of credited service considered in determining retirement benefit	40 years
Number of participants in the Pension Plan	1,005
Defined contribution plan
Percentage of participant's eligible pay contributed to the plan by the employer		5.25%
Eligibility for employer matching contributions, period of service		3 years
Employer contribution to the plan	$ 254,000	$ 85,000
Post-Retirement Medical Benefits
Employee benefit plans
Number of participants in the Pension Plan	307