Fair Value (Reconciliation of Assets Measured at Fair Value on a Recurring Basis Using Level 3 Inputs) (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 488	$ 388	$ 500	$ 384
Purchases	8	16	24	24
Sales	(2)	(15)	(11)	(16)
Net unrealized (losses) gains in accumulated other comprehensive income	(5)	1	(8)	(3)
Net realized gains (losses) in investment and other income	0	13	(16)	14
Balance at end of period	489	403	489	403
Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	98	79	98	74
Purchases	0	6	4	10
Sales	0	(2)	(7)	(2)
Net unrealized (losses) gains in accumulated other comprehensive income	3	1	6	2
Net realized gains (losses) in investment and other income	0	0	0	0
Balance at end of period	101	84	101	84
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	390	309	402	310
Purchases	8	10	20	14
Sales	(2)	(13)	(4)	(14)
Net unrealized (losses) gains in accumulated other comprehensive income	(8)	0	(14)	(5)
Net realized gains (losses) in investment and other income	0	13	(16)	14
Balance at end of period	$ 388	$ 319	$ 388	$ 319