Condensed Financial Information (Parent Company) (Interest Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest Expense [Abstract]
Total interest expense	$ 841	$ 874	$ 890
Parent Company
Interest Expense [Abstract]
Total interest expense	1,108	1,152	1,171
Parent Company | Long-term Debt
Interest Expense [Abstract]
Total interest expense	774	811	833
Parent Company | Long-term Debt | Affiliated Entity [Member]
Interest Expense [Abstract]
Total interest expense	112	160	168
Parent Company | Secured Debt [Member]
Interest Expense [Abstract]
Total interest expense	27	47	36
Parent Company | Junior Subordinated Debt [Member]
Interest Expense [Abstract]
Total interest expense	$ 195	$ 134	$ 134