Long-Term Debt (Restrictions and Covenants) (Details) (USD $)	Dec. 31, 2012
Debt Instrument [Line Items]
The maximum threshold of letters of credit excluded from the leverage ratio calculation	$ 150,000,000
Capitalization ratio calculation, additions to shareholders' equity	$ 1,520,000,000
Maximum Ratio Requirement [Member]
Debt Instrument [Line Items]
Capitalization ratio	65.00%
Leverage ratio	3.75
Debt Covenant, Actual Ratio [Member]
Debt Instrument [Line Items]
Capitalization ratio	59.10%
Leverage ratio	2.82