Income Taxes (Narrative) (Details) $ in Thousands			12 Months Ended			60 Months Ended
		Jan. 01, 2018 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) generator	Dec. 31, 2018 USD ($) fund generator	Dec. 31, 2017 generator
Income Tax Contingency [Line Items]
Income tax expense			$ 117,968	$ 169,882	$ 114,995
Effective tax rate (as a percent)			17.20%	20.10%	14.00%
Valuation allowance			$ 0	$ 21
Deferred tax assets, net			163,815	106,487
DC Solar Tax Credit Investments
Income Tax Contingency [Line Items]
Number of tax credit funds | fund						4
Claimed investment tax credits						$ 53,900
Deferred tax liability reduction						5,700
Net impact to Consolidated Financial Statements						$ 48,200
Mobile service generator | generator					0	0	0
Amount reversed from previously claimed tax credits				33,600
Additional income tax expense				30,100
Amount reversed from deferred tax liability				$ 3,500
Uncertain Tax Position Recorded in Income Tax Expense			$ 5,100
Accounting Standards Update 2018-02
Income Tax Contingency [Line Items]
Reclassification of tax effects in AOCI resulting from the new federal corporate income tax rate		$ 6,700
Retained Earnings | Accounting Standards Update 2018-02
Income Tax Contingency [Line Items]
Reclassification of tax effects in AOCI resulting from the new federal corporate income tax rate	[1]				$ 6,656
AOCI, net of Tax | Accounting Standards Update 2018-02
Income Tax Contingency [Line Items]
Reclassification of tax effects in AOCI resulting from the new federal corporate income tax rate	[1]				$ (6,656)

[1] Represents amounts reclassified from AOCI to retained earnings due to the early adoption of ASU 2018-02, Income Statement — Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income on January 1, 2018.