Shareholders' Equity, Stock Repurchase Program, Dividends and Issuance of Common Stock (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Equity, Class of Treasury Stock [Line Items]
Approved funds available for common stock repurchase program	$ 20
Stock repurchase program, period in force	3 years
Number of shares of stock repurchased (in shares)	0	0
Retained net profit period	2 years
Issuance of common stock (in shares)	4,975	16,542
Issuance of common stock for contribution to non-qualified defined contribution retirement plans (in shares)	4,975	4,610
Minimum [Member]
Equity, Class of Treasury Stock [Line Items]
Per share price of shares issued (in dollars per share)	$ 590	$ 525
Maximum [Member]
Equity, Class of Treasury Stock [Line Items]
Per share price of shares issued (in dollars per share)	$ 595	$ 580
Delta National Bancorp [Member]
Equity, Class of Treasury Stock [Line Items]
Issuance of common stock in acquisition (in shares)		11,932