ARO Trust (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
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	55.0	42.8
Available-for-sale securities, fair value	60.3	47.5
Cash and Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost basis	3.1	2.1
Available-for-sale securities, fair value	3.1	2.1
U.S. Equity Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost basis	18.0	15.0
Available-for-sale securities, fair value	22.3	19.0
International Equity Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost basis	11.0	8.0
Available-for-sale securities, fair value	11.5	8.2
Municipal Bond Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost basis	22.9	17.7
Available-for-sale securities, fair value	$ 23.4	$ 18.2