Derivative Instrument and Hedging Activities - Narrative (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2011
Derivative [Line Items]
Derivative, Net Liability Position, Aggregate Fair Value	$ 3
Term Loan due 2015 [Member]
Derivative [Line Items]
Debt, face amount	455
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative notional amount	525
Derivative fixed interest rate	3.63%
Interest Rate Swap [Member] | Term Loan due 2015 [Member]
Derivative [Line Items]
Derivative notional amount	455
Interest Rate Swap [Member] | Interest Rate Swap, Ten Year Period [Member]
Derivative [Line Items]
Derivative notional amount	450
Interest rate swap, period (in years)		10 years
Interest Rate Swap [Member] | Interest Rate Swap, Five Year Period [Member]
Derivative [Line Items]
Derivative notional amount	$ 75
Interest rate swap, period (in years)		5 years
Interest Rate Swap [Member] | LIBOR [Member] | Junior Subordinated Debt [Member]
Derivative [Line Items]
Debt instrument, three month LIBOR, basis spread on variable rate	3.0175%
Natural gas price swaps [Member] | Year 2013 [Member] | MMBtus [Member] | Distribution [Member]
Derivative [Line Items]
Derivative notional amount, nonmonetary (in MMBtu or gallons)	8,300,000
Natural gas price swaps [Member] | Year 2014 [Member] | MMBtus [Member] | Distribution [Member]
Derivative [Line Items]
Derivative notional amount, nonmonetary (in MMBtu or gallons)	15,110,000
Natural gas price swaps [Member] | Year 2015 [Member] | MMBtus [Member] | Distribution [Member]
Derivative [Line Items]
Derivative notional amount, nonmonetary (in MMBtu or gallons)	4,470,000