Fair Value Measurements - Quantative Information about Level 3 Fair Value (Details) - Jun. 30, 2015	Total
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	2.00%
Discounted Cash Flow | Time To Principal Recovery | Auction rate securities
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range Min, Term	8 years
Third Quarter of 2015 | Option Model | Implied Volatility | Fuel derivatives
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum	18.00%
Fair Value Measurement Range, Maximum	33.00%
Fourth Quarter of 2015 | Option Model | Implied Volatility | Fuel derivatives
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum	26.00%
Fair Value Measurement Range, Maximum	35.00%
2016 | Option Model | Implied Volatility | Fuel derivatives
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum	23.00%
Fair Value Measurement Range, Maximum	37.00%
2017 | Option Model | Implied Volatility | Fuel derivatives
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum	17.00%
Fair Value Measurement Range, Maximum	29.00%
2018 | Option Model | Implied Volatility | Fuel derivatives
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum	19.00%
Fair Value Measurement Range, Maximum	23.00%