Income Taxes (Tax Carryforwards) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Gross	$ 189,445	$ 432,494
Deferred Tax Assets, Valuation Allowance	(3,431)	(9,658)
Total deferred tax assets	186,014	$ 422,836
Alternative minimum tax credits	42,100
State | Net Operating Loss | 2023-2027
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Gross	1,688
Deferred Tax Assets, Valuation Allowance	0
Total deferred tax assets	1,688
Pre-Tax Earnings Necessary to Realize(1)	1,018,310
State | Net Operating Loss | 2028-2032
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Gross	48,430
Deferred Tax Assets, Valuation Allowance	0
Total deferred tax assets	48,430
Pre-Tax Earnings Necessary to Realize(1)	1,093,946
State | Net Operating Loss | 2033-2035
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Gross	360
Deferred Tax Assets, Valuation Allowance	0
Total deferred tax assets	360
Pre-Tax Earnings Necessary to Realize(1)	7,584
State | Other Credits
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Gross	587
Deferred Tax Assets, Valuation Allowance	0
Total deferred tax assets	587
State | Other Credits | 2018-2022
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Gross	18,381
Deferred Tax Assets, Valuation Allowance	(3,431)
Total deferred tax assets	14,950
State | Other Credits | 2023-2027
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Gross	207
Deferred Tax Assets, Valuation Allowance	0
Total deferred tax assets	$ 207