Income Taxes (Tax Carryforwards) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Gross	$ 547,890	$ 672,840
Deferred Tax Assets, Valuation Allowance	(11,713)	(12,303)
Total deferred tax assets	536,177	$ 660,537
Alternative Minimum Tax Credit [Member]
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Gross	31,745
Deferred Tax Assets, Valuation Allowance	0
Total deferred tax assets	31,745
Federal [Member] | Net Operating Loss [Member] | 2030-2034 [Member]
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Gross	256,807
Deferred Tax Assets, Valuation Allowance	0
Total deferred tax assets	256,807
Pre-Tax Earnings Necessary to Realize	733,734
Federal [Member] | General Business Tax Credits [Member] | 2028-2035 [Member]
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Gross	11,667
Deferred Tax Assets, Valuation Allowance	0
Total deferred tax assets	11,667
State [Member] | Net Operating Loss [Member] | 2016-2019 [Member]
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Gross	10
Deferred Tax Assets, Valuation Allowance	0
Total deferred tax assets	10
Pre-Tax Earnings Necessary to Realize	689,362
State [Member] | Net Operating Loss [Member] | 2024-2028 [Member]
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Gross	4,031
Deferred Tax Assets, Valuation Allowance	0
Total deferred tax assets	4,031
Pre-Tax Earnings Necessary to Realize	660,830
State [Member] | Net Operating Loss [Member] | 2029-2035 [Member]
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Gross	54,674
Deferred Tax Assets, Valuation Allowance	0
Total deferred tax assets	54,674
Pre-Tax Earnings Necessary to Realize	1,744,165
State [Member] | Other Credits [Member]
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Gross	1,445
Deferred Tax Assets, Valuation Allowance	0
Total deferred tax assets	1,445
State [Member] | Other Credits [Member] | 2016-2019 [Member]
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Gross	11,800
Deferred Tax Assets, Valuation Allowance	(11,713)
Total deferred tax assets	87
State [Member] | Other Credits [Member] | 2020-2025 [Member]
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Gross	2,777
Deferred Tax Assets, Valuation Allowance	0
Total deferred tax assets	$ 2,777