Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Marketable and Non-Marketable Securities
Net unrealized gains (losses) recorded in accumulated other comprehensive income (loss)	$ 1.0	$ 2.6
Carrying value of investments under the cost method	118.8	98.9
Other-than-temporary non-cash charges	0
Available for sale securities | Securities in captive insurance subsidiary portfolio | Minimum
Marketable and Non-Marketable Securities
Investment maturity period	1 year
Available for sale securities | Securities in captive insurance subsidiary portfolio | Maximum
Marketable and Non-Marketable Securities
Investment maturity period	10 years
Held-to-maturity securities
Marketable and Non-Marketable Securities
Investments used to fund debt service requirements	$ 16.5	$ 24.9