Credit Agreement (Details) (Bank Of America February Two Thousand Eleven New Credit Agreement [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 covenants	Feb. 08, 2011
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 200
Option to increase maximum borrowing capacity		150
Term of agreement		5 years
Eurodollar interest rate, variable interest rate lower limit	1.805%
Prime lending rate, variable interest rate upper limit	3.25%
Commitment fee rate	0.25%
Covenant terms	The Credit Agreement contains two financial covenants that require the maintenance of certain leverage ratios.
Letters of credit outstanding	$ 10.6
Letter of Credit Facility, Number of Covenants	2
Minimum [Member]
Line of Credit Facility [Line Items]
Commitment fee rate	0.25%
Maximum [Member]
Line of Credit Facility [Line Items]
Commitment fee rate	0.40%