Basis of Presentation and Significant Accounting Policies Noncontrolling Interest (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Jun. 30, 2018 USD ($) shares
Stockholders' Equity Attributable to Noncontrolling Interest | $	$ 60	$ 60	$ 264
Preferred Stock, Shares Outstanding | shares	0	0
Ambac Assurance Corporation [Member]
Preferred Stock, Shares Outstanding | shares	5,501	5,501	26,411
Preferred Stock, Liquidation Preference, Value | $	$ 138	$ 138	$ 660
London Interbank Offered Rate (LIBOR) [Member]
Dividend Rate on Auction Rate Preferred Shares	200