Income Taxes (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Contingency [Line Items]
Impact of Tax Reform	$ 172,100
Deferred Tax Assets, Operating Loss Carryforwards, Domestic	41,282	$ 187,817
Gross unrecognized tax benefits	48,604	21,502	$ 38,992	$ 32,911
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	23,400	14,000
Unrecognized tax benefits, income tax penalties and interest accrued	4,900	12,200
Possible decrease in unrecognized tax benefits	$ 40,000
Minimum [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards, expiration dates	Jan. 01, 2031
IncomeTaxNetOperatingLossCarryforwardPeriod	5 years
Income tax examination, year(s) under examination	2005
Maximum [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards, expiration dates	Jan. 01, 2032
IncomeTaxNetOperatingLossCarryforwardPeriod	20 years
Income tax examination, year(s) under examination	2017
2016 to 2020 [Member] | Minimum [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards, expiration dates	Jan. 01, 2018
2016 to 2020 [Member] | Maximum [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards, expiration dates	Jan. 01, 2022
2016 to 2020 [Member] | State and Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards, Subject to Expiration	$ 26,900
2021 to 2025 [Member] | Minimum [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards, expiration dates	Jan. 01, 2023
2021 to 2025 [Member] | State and Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards, Subject to Expiration	$ 221,300
Net Operating Loss (NOL) Expiration Period energy credit [Member] | Minimum [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards, expiration dates	Jan. 01, 2027
Net Operating Loss (NOL) Expiration Period energy credit [Member] | Maximum [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards, expiration dates	Jan. 01, 2036
New Accounting Pronouncement, Early Adoption, Effect [Member] | Retained Earnings [Member] | Accounting Standards Update 2016-09, Excess Tax Benefit Component [Member]
Income Tax Contingency [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption		$ 18,600