ARO Trust (Details) (USD $) In Millions, unless otherwise specified	Dec. 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	42.8	28.9
Available-for-sale securities, fair value	47.5	32.7
Cash And Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost basis	2.1	6.5
Available-for-sale securities, fair value	2.1	6.5
U.S. Equity Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost basis	15.0	8.0
Available-for-sale securities, fair value	19.0	11.1
International Equity Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost basis	8.0	4.2
Available-for-sale securities, fair value	8.2	4.9
Municipal Bond Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost basis	17.7	10.2
Available-for-sale securities, fair value	$ 18.2	$ 10.2