Shareholders' Equity, Stock Repurchase Program, Dividends and Issuance of Common Stock (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Equity, Class of Treasury Stock [Line Items]
Approved funds available for common stock repurchase program	$ 20
Stock repurchase program, period in force	3 years
Retained net profit period	2 years
Issuance of common stock (in shares)	16,542	6,705
Issuance of common stock for contribution to non-qualified defined contribution retirement plans (in shares)	4,610	6,705
Minimum [Member]
Equity, Class of Treasury Stock [Line Items]
Per share price of shares issued (in dollars per share)	$ 525	$ 450
Maximum [Member]
Equity, Class of Treasury Stock [Line Items]
Per share price of shares issued (in dollars per share)	$ 580	$ 525
Delta National Bancorp [Member]
Equity, Class of Treasury Stock [Line Items]
Issuance of common stock in acquisition (in shares)	11,932