Investment Securities (Investments, by type and length in continuous loss position) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Dec. 31, 2013
Available-for-sale Securities Estimated Fair Value And Gross Unrealized Losses [Abstract]
Estimated fair value, less than 12 months	$ 4,052	$ 2,255
Gross unrealized losses, less than 12 months	(164)	(22)
Estimated fair value, 12 months or more	645	1,554
Gross unrealized losses, 12 months or more	$ (121)	(136)
Percent Of Gross Unrealized Losses, 12 Months Or More, Considered Investment Grade	97.00%
OTTI Previously Recognized Through OCI On Securities Held, Gross Unrealized Losses	$ 27
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount			$ 0
US Corporate
Available-for-sale Securities Estimated Fair Value And Gross Unrealized Losses [Abstract]
Estimated fair value, less than 12 months	2,161	554
Gross unrealized losses, less than 12 months	(130)	(16)
Estimated fair value, 12 months or more	335	836
Gross unrealized losses, 12 months or more	(48)	(53)
State and municipal
Available-for-sale Securities Estimated Fair Value And Gross Unrealized Losses [Abstract]
Estimated fair value, less than 12 months	544	67
Gross unrealized losses, less than 12 months	(16)	(1)
Estimated fair value, 12 months or more	155	308
Gross unrealized losses, 12 months or more	(55)	(52)
Residential Mortgage Backed Securities [Member]
Available-for-sale Securities Estimated Fair Value And Gross Unrealized Losses [Abstract]
Estimated fair value, less than 12 months	175	30
Gross unrealized losses, less than 12 months	(2)	0
Estimated fair value, 12 months or more	77	146
Gross unrealized losses, 12 months or more	(3)	(5)
Residential Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member] | Residential Mortgage Backed Securities [Member]
Available-for-sale Securities Estimated Fair Value And Gross Unrealized Losses [Abstract]
Debt securities, Estimated fair value	23
Residential Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Residential Mortgage Backed Securities [Member]
Available-for-sale Securities Estimated Fair Value And Gross Unrealized Losses [Abstract]
Debt securities, Estimated fair value	961
Mortgage-backed Securities, Residential, Subprime, Financing Receivable [Member] | Residential Mortgage Backed Securities [Member]
Available-for-sale Securities Estimated Fair Value And Gross Unrealized Losses [Abstract]
Debt securities, Estimated fair value	58
Commercial mortgage backed
Available-for-sale Securities Estimated Fair Value And Gross Unrealized Losses [Abstract]
Estimated fair value, less than 12 months	351	165
Gross unrealized losses, less than 12 months	(8)	(1)
Estimated fair value, 12 months or more	26	204
Gross unrealized losses, 12 months or more	(4)	(9)
Asset-backed
Available-for-sale Securities Estimated Fair Value And Gross Unrealized Losses [Abstract]
Estimated fair value, less than 12 months	0	9
Gross unrealized losses, less than 12 months	0	0
Estimated fair value, 12 months or more	48	42
Gross unrealized losses, 12 months or more	(10)	(17)
Corporate non US
Available-for-sale Securities Estimated Fair Value And Gross Unrealized Losses [Abstract]
Estimated fair value, less than 12 months	41	42
Gross unrealized losses, less than 12 months	(3)	(1)
Estimated fair value, 12 months or more	3	3
Gross unrealized losses, 12 months or more	0	0
Government - non-U.S.
Available-for-sale Securities Estimated Fair Value And Gross Unrealized Losses [Abstract]
Estimated fair value, less than 12 months	292	677
Gross unrealized losses, less than 12 months	(1)	(2)
Estimated fair value, 12 months or more	0	14
Gross unrealized losses, 12 months or more	0	0
U.S. Government and federal agency
Available-for-sale Securities Estimated Fair Value And Gross Unrealized Losses [Abstract]
Estimated fair value, less than 12 months	450	705
Gross unrealized losses, less than 12 months	0	0
Estimated fair value, 12 months or more	1	1
Gross unrealized losses, 12 months or more	0	0
Equity Securities [Member]
Available-for-sale Securities Estimated Fair Value And Gross Unrealized Losses [Abstract]
Estimated fair value, less than 12 months	38	6
Gross unrealized losses, less than 12 months	(4)	(1)
Estimated fair value, 12 months or more	0	0
Gross unrealized losses, 12 months or more	0	$ 0
Retained Interest [Member]
Available-for-sale Securities Estimated Fair Value And Gross Unrealized Losses [Abstract]
Estimated fair value, less than 12 months	0
Gross unrealized losses, less than 12 months	0
Estimated fair value, 12 months or more	0
Gross unrealized losses, 12 months or more	$ 0