Loans and Allowance for Loan Losses - Purchased Credit-Impaired Loans Segment (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Aging of loans
Loans covered under a loss share agreement with FDIC		$ 19	$ 126
Commercial Loan
Aging of loans
Loans covered under a loss share agreement with FDIC		1	98
Total loans		46,505	44,066
Commercial portfolio
Aging of loans
Total loans		46,392	43,863
Commercial portfolio | Commercial and industrial
Aging of loans
Total loans		30,337	28,283
Commercial portfolio | Construction
Aging of loans
Total loans		2,297	1,747
Commercial portfolio | Commercial mortgage
Aging of loans
Total loans		13,758	13,833
Purchased credit-impaired loans | Commercial Loan
Aging of loans
Total loans		113	203
Pass | Commercial Loan
Aging of loans
Total loans		43,978	42,759
Pass | Commercial portfolio
Aging of loans
Total loans		43,938	42,722
Pass | Commercial portfolio | Commercial and industrial
Aging of loans
Total loans		28,228	27,471
Pass | Commercial portfolio | Construction
Aging of loans
Total loans		2,240	1,729
Pass | Commercial portfolio | Commercial mortgage
Aging of loans
Total loans		13,470	13,522
Pass | Purchased credit-impaired loans | Commercial Loan
Aging of loans
Total loans		40	37
Special Mention | Commercial Loan
Aging of loans
Total loans		1,052	636
Special Mention | Commercial portfolio
Aging of loans
Total loans		1,040	598
Special Mention | Commercial portfolio | Commercial and industrial
Aging of loans
Total loans		844	452
Special Mention | Commercial portfolio | Construction
Aging of loans
Total loans		57	18
Special Mention | Commercial portfolio | Commercial mortgage
Aging of loans
Total loans		139	128
Special Mention | Purchased credit-impaired loans | Commercial Loan
Aging of loans
Total loans		12	38
Classified | Commercial Loan
Aging of loans
Total loans		1,475	671
Classified | Commercial portfolio
Aging of loans
Total loans		1,414	543
Classified | Commercial portfolio | Commercial and industrial
Aging of loans
Total loans		1,265	360
Classified | Commercial portfolio | Construction
Aging of loans
Total loans		0	0
Classified | Commercial portfolio | Commercial mortgage
Aging of loans
Total loans		149	183
Classified | Purchased credit-impaired loans | Commercial Loan
Aging of loans
Total loans		61	$ 128
Consumer Loan
Aging of loans
Proceeds from Sale and Collection of Loans Held-for-sale	$ 827
Oil and gas | Criticized [Member] | Commercial portfolio
Aging of loans
Total loans		$ 1,226