Reconciliation of Income Tax Expense from Continuing Operations (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Income Tax Rate Reconciliation [Line Items]      
Income tax at statutory rate $ 211,914 $ 220,940 $ 176,288
State taxes, net of federal taxes 21,852 32,877 24,027
Change in valuation allowance (455) 143 (160)
Flow through differences 3,217 3,032 2,895
Amortization of deferred investment tax credits (1,501) (1,518) (1,542)
Subsidiary preferred dividends 584 634 668
Other, net 595 900 (3,425)
Actual income tax expense $ 236,206 $ 257,008 $ 198,751