Subsequent Events (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate, LIBOR Loans	2.00%
Debt Instrument, Basis Spread on Variable Rate, Base Rate Loans	1.00%
Letters of Credit Outstanding, Amount	$ 4.2
Letters of Credit Outstanding, Amount, Subsequent	7.5
Consolidated Fixed Charge Coverage Ratio, Minimum, Collateral Release Period	1.25
Consolidated Leverage Ratio, Maximum, Collateral Release Period	2.50
Consolidated Tangible Net Worth, Actual	791.8
Consolidated Tangible Net Worth, Covenant	666.6
Consolidated Interest Coverage Ratio, Covenant	4.00
Consolidated Leverage Ratio, Covenant	3.25
Consolidated Leverage Ratio, Maximum, Quarter Ending December 31, 2013	3.00
Senior Notes	208.3	216.7	216.7
Minimum [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate at Period End	2.00%
Expected Gain	17.0
Maximum [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate at Period End	4.00%
Expected Gain	18.0
Institutional Group One [Member]
Debt Instrument [Line Items]
Consolidated Leverage Ratio, Covenant	3.50
Senior Notes	8.3
Debt Instrument, Interest Rate, Stated Percentage	7.00%
Institutional Group Two [Member]
Debt Instrument [Line Items]
Senior Notes	200.0
Debt Instrument, Interest Rate, Stated Percentage	6.10%
Institutional Groups One and Two [Member]
Debt Instrument [Line Items]
Consolidated Leverage Ratio, Maximum, Collateral Release Period	2.50
Consolidated Tangible Net Worth, Actual	791.8
Consolidated Tangible Net Worth, Covenant	666.6
Consolidated Interest Coverage Ratio, Actual	11.24
Consolidated Interest Coverage Ratio, Covenant	4.00
Consolidated Leverage Ratio, Actual	1.44
Consolidated Leverage Ratio, Maximum, Quarter Ending December 31, 2013	3.25
Letter of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	100.0
Line of Credit Facility, Maximum Borrowing Capacity, Previous	100.0
Line of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 215.0