Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Contingency [Line Items]
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Impairment Losses, Amount	$ 524.0	$ 72.0	$ 608.0
Valuation allowance	605.0	497.0
Effect of expiration of statute of limitations during next twelve months	0.0
Gross tax liability for tax positions that may not be sustained under a more likely than not threshold	12.0
Expected income refunds	1.0
Income taxes paid	4.0	$ 4.0
Research Tax Credit Carryforward [Member]
Income Tax Contingency [Line Items]
Tax Credit Carryforward, Amount	$ 21.0
Shareholder's Right Plan [Member]
Income Tax Contingency [Line Items]
Beneficial ownership	4.90%
State and Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Tax Credit Carryforward, Amount	$ 36.0
Operating loss carryforward	$ 9,800.0
Tax credit expiration year	2039
Internal Revenue Service (IRS) [Member]
Income Tax Contingency [Line Items]
Operating loss carryforward	$ 1,600.0
Tax credit expiration year	2036
Internal Revenue Service (IRS) [Member] | Indefinite Tax Period [Member]
Income Tax Contingency [Line Items]
Operating loss carryforward	$ 202.0
Minimum [Member] | State and Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Tax credit expiration year	2020
Minimum [Member] | Internal Revenue Service (IRS) [Member]
Income Tax Contingency [Line Items]
Tax credit expiration year	2034
Maximum [Member] | State and Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Tax credit expiration year	2023
Maximum [Member] | Internal Revenue Service (IRS) [Member]
Income Tax Contingency [Line Items]
Tax credit expiration year	2039