Long-Term Debt and Capital Leases (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 numberofbanks	Dec. 31, 2011
Maturities of Long-term Debt [Abstract]
2013	$ 1.6
2014	1.7
2015	1.7
2016	151.8
2017	2.0
Unsecured Revolving Line Of Credit [Member] | Line of Credit [Member]
Unsecured Revolving Line of Credit Facility [Line Items]
Current borrowing capacity	300
Maximum borrowing capacity	350
Number of institutions participating in the credit faciltiy	8
Maximum number of institutions participating in the credit faciltiy pertaining to maximum contributory percentage	1
Letters of credit outstanding, amount	3.5
Debt related commitment fees	$ 0.5	$ 0.7
Covenant description, debt instrument,	maximum debt to capitalization ratio
Unsecured Revolving Line Of Credit [Member] | Line of Credit [Member] | Minimum [Member] | LIBOR Plus [Member]
Unsecured Revolving Line of Credit Facility [Line Items]
Basis spread on variable rate	0.88%
Unsecured Revolving Line Of Credit [Member] | Line of Credit [Member] | Maximum [Member]
Unsecured Revolving Line of Credit Facility [Line Items]
Maximum percentage of total availability provided by a single lender	17.00%
Maximum ratio of indebtedness to net capital threshold percentage	65.00%
Unsecured Revolving Line Of Credit [Member] | Line of Credit [Member] | Maximum [Member] | LIBOR Plus [Member]
Unsecured Revolving Line of Credit Facility [Line Items]
Basis spread on variable rate	1.75%