Income Taxes (Narrative) (Details) $ in Thousands			12 Months Ended			15 Months Ended	60 Months Ended
		Jan. 01, 2018 USD ($)	Dec. 31, 2019 USD ($) generator	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) fund
Income Tax Contingency [Line Items]
Income tax expense related to impact of the Tax Act				$ 985	$ 41,700	$ 42,700
Remeasurements of certain deferred tax assets and liabilities, net					33,100
Remeasurements of tax credits and other tax benefits related to qualified affordable housing partnerships					$ 7,900
Decrease in tax credits recognized from investments			$ 14,700
Increase in effective tax rate (as a percent)			6.10%
Effective tax rate (as a percent)			20.10%	14.00%	31.20%
Valuation allowance			$ 21	$ 128		128	$ 128
Deferred tax assets, net			$ 106,487	$ 117,640		$ 117,640	$ 117,640
Reclassification of tax effects in AOCI resulting from the new federal corporate income tax rate	[1]	$ 0
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
Amount reversed from previously claimed tax credits			$ 33,600
Amount reversed from deferred tax liability			3,500
Additional income tax expense			$ 30,100
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		6,656
Retained Earnings
Income Tax Contingency [Line Items]
Reclassification of tax effects in AOCI resulting from the new federal corporate income tax rate	[1]	6,656
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		6,656
AOCI, net of Tax
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		$ (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 in the first quarter of 2018. Refer to Note 1 — Summary of Significant Accounting Policies to the Consolidated Financial Statements for additional information.