Significant Credit Concentrations (Details) - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Maximum Credit Exposure by Category
On-balance sheet	$ 150	$ 147
Individuals [Member]
Maximum Credit Exposure by Category
On-balance sheet	104	101
Unused lines-of-credit	297	278
Financial Institutions [Member]
Maximum Credit Exposure by Category
On-balance sheet	25	25
United States Government And Agencies [Member]
Maximum Credit Exposure by Category
On-balance sheet	4	4
Other Concentration [Member]
Maximum Credit Exposure by Category
On-balance sheet	$ 17	$ 17