Debt Line of Credit Facility (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Apr. 30, 2011
April 2011 Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Initiation Date	April 2011
Line of Credit Facility, Term	five year
Line of Credit Facility, Maximum Borrowing Capacity Before Increase	300
Line of Credit Facility, Maximum Borrowing Capacity	450
Line of Credit Facility, Interest Rate Description	LIBOR plus 105 basis points
Line of Credit Facility, Commitment Fee Description	20 basis points
August 2006 Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		250
Debt Instrument, Maturity Date, Description	August 2011
April 2011 Line of Credit as Amended October 2012 [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Initiation Date	October 2012
Debt Instrument, Maturity Date, Description	April 2016
Line of Credit Facility, Interest Rate Description	LIBOR plus 97.5 basis points
Line of Credit Facility, Commitment Fee Description	15 basis points
Line of Credit Facility, Remaining Borrowing Capacity	171
Line of Credit Facility, Amount to Secure Standby Letters of Credit	4