Fair Value Measurements - Quantative Information about Level 3 Fair Value (Details)	Sep. 30, 2013
Discounted Cash Flow | Illiquidity Premium | Auction rate securities
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum	3.00%
Fair Value Measurement Range, Maximum	4.00%
Discounted Cash Flow | Counterparty Credit Spread | Auction rate securities
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum	1.00%
Fair Value Measurement Range, Maximum	3.00%
Discounted Cash Flow | Time To Principal Recovery | Auction rate securities
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum, Term	5 years
Fair Value Measurement Range, Maximum, Term	8 years
Fourth Quarter of 2013 | Option Model | Implied Volatility | Fuel derivatives
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum	11.00%
Fair Value Measurement Range, Maximum	26.00%
2014 | Option Model | Implied Volatility | Fuel derivatives
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum	14.00%
Fair Value Measurement Range, Maximum	28.00%
2015 | Option Model | Implied Volatility | Fuel derivatives
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum	14.00%
Fair Value Measurement Range, Maximum	22.00%
2016 | Option Model | Implied Volatility | Fuel derivatives
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum	14.00%
Fair Value Measurement Range, Maximum	20.00%
2017 | Option Model | Implied Volatility | Fuel derivatives
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum	14.00%
Fair Value Measurement Range, Maximum	17.00%