Debt Credit Facility Narrative (Details) (USD $)										12 Months Ended
	Mar. 31, 2012	Dec. 31, 2012 Credit Facility Term Loan [Member]	Jun. 26, 2012 Credit Facility Term Loan [Member]	Mar. 31, 2012 Credit Facility Term Loan [Member]	Jun. 30, 2011 Preceding Credit Facility Revolver [Member]	Dec. 31, 2012 Revolving Credit Facility [Member]	Mar. 31, 2012 Revolving Credit Facility [Member]	Dec. 31, 2012 Letter of Credit [Member]	Mar. 31, 2012 Letter of Credit [Member]	Dec. 31, 2012 Through June 30, 2013 [Member] Credit Facility Term Loan [Member]	Dec. 31, 2012 September 30, 2013 through March 31, 2016 [Member] Credit Facility Term Loan [Member]
Debt Instrument [Line Items]
Term loan original issuance amount			$ 330,000,000
Revolver maximum borrowing capacity					150,000,000
Debt Instrument, Interest Rate, Stated Percentage	4.25%
Quarterly principal payment of Term Loan										4,000,000	8,000,000
Interest rate on Term Loan		5.00%		3.50%
Revolver Amount Outstanding						40,000,000
Letters of credit outstanding, amount								22,000,000	21,000,000
Revolver remaining borrowing capacity						$ 88,000,000	$ 129,000,000