Employee Benefit Plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011 item	Dec. 31, 2010
Employee Benefit Plans
Percentage of contribution of covered employee's salaries or wages	2.50%
Contribution	$ 5.2	$ 5.1	$ 4.3
Amount payable to plan	0.1	0.1
Number of employees who are union members	5	7
Period in which certain individuals are entitled to fixed annual payments	15 years
Portion of benefits vesting after ten years of completed service (as a percent)	50.00%
Period of completed service over which 50% of benefits are vested	10 years
Portion of benefits vesting after fifteen years of completed service (as a percent)	75.00%
Period of completed service over which 75% of benefits are vested	15 years
Period of service over which benefits are fully vested	20 years
Unfunded benefit liability	$ 3.0	$ 2.8