Debt (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended			1 Months Ended	0 Months Ended	12 Months Ended				12 Months Ended		12 Months Ended
	Nov. 21, 2011 years	Dec. 31, 2011 days	Dec. 31, 2010	Dec. 31, 2009	Nov. 21, 2011 Letter Of Credit [Member]	Nov. 21, 2011 Federal Funds Rate [Member]	Dec. 31, 2011 Euro Dollar Rate Option [Member]	Dec. 31, 2011 Base Rate Option [Member]	Dec. 31, 2011 Credit Facility [Member]	Dec. 31, 2011 Minimum [Member] Euro Dollar Rate Option [Member]	Dec. 31, 2011 Minimum [Member] Credit Facility [Member]	Dec. 31, 2011 Maximum [Member] Euro Dollar Rate Option [Member]	Dec. 31, 2011 Maximum [Member] Credit Facility [Member]
Debt Instrument [Line Items]
Finance through unsecured revolving credit facility	$ 150.0
Maturity period of unsecured revolving credit facility, in years	5
Aggregate total finance under Credit Facility	225.0
Expiration date of unsecured revolving credit facility	November 21, 2016
Increase in the total financing available under the Credit Facility	75.0
Maximum letter of credit commitment					20.0
Outstanding letters of credit		0.2
Outstanding borrowings under the credit facility			53.0
Interest rate description under credit facility							Borrowings under the Eurodollar option bear interest at the London Interbank Offered Rate (LIBOR) plus a margin of 1.05% to 1.40%.	Borrowings under the Eurodollar option bear interest at the London Interbank Offered Rate (LIBOR) plus a margin of 1.05% to 1.40%. Borrowings under the base rate option bear interest at the higher of the lead lender's prime rate, the Federal Funds rate plus 50 basis points, or the one-month LIBOR rate adjusted for the statutory reserve rate plus 1%.	The Credit Facility contains a facility fee provision whereby the Company is charged a fee, ranging from 0.20% to 0.35%, applied to the total amount of the commitment.
Federal funds rate basis points						50
Interest rate margin adjustment in days		90
Borrowings under base rate plus margin		1.00%								1.05%		1.40%
Percentage of commitment fee on credit facility											0.20%		0.35%
Interest paid		0.5	0.9	1.4
Interest cost capitalized		0.1	0.1	0.1
Amortization of deferred financing cost		$ 0.1	$ 0.1	$ 0.1
Interest rate on outstanding borrowings			0.83%