Variable Interest Entities (Narrative) (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Variable Interest Entity [Line Items]
Gains (losses) for consolidated variable interest entities	$ 0		$ 0
Carrying Amounts of our Investments in LPs and LLCs, As Recognized In Other Investments on our Consolidated Balance Sheets	2,049,000,000		2,296,000,000
Income Tax Credits And Other Tax Benefits From Qualified Affordable Housing Projects	1,000,000	$ 1,000,000
Limited Partnerships and Limited Liability Companies [Member]
Variable Interest Entity [Line Items]
Carrying Amounts of our Investments in LPs and LLCs, As Recognized In Other Investments on our Consolidated Balance Sheets	1,600,000,000		1,500,000
Carrying Amount Of Investments In Qualified Affordable Housing Projects	$ 29,000,000		$ 31,000,000