ARO Trust (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Investments, Debt and Equity Securities [Abstract]
Annual funding obligation	$ 36.4
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost basis	41.5	28.9
Available-for-sale securities, fair value	45.0	32.7
Cash and Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost basis	13.9	6.5
Available-for-sale securities, fair value	13.9	6.5
U.S. Equity Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost basis	6.7	8.0
Available-for-sale securities, fair value	9.3	11.1
International Equity Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost basis	5.7	4.2
Available-for-sale securities, fair value	6.4	4.9
Municipal Bond Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost basis	15.2	10.2
Available-for-sale securities, fair value	$ 15.4	$ 10.2