Consolidated Statements of Changes In Shareholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Statement of Stockholders' Equity [Abstract]
Cash dividends (in dollars per share)	$ 1.25	$ 1.20
Stock issuance costs	$ 0	$ 0
Class A Common Stock
Statement of Stockholders' Equity [Abstract]
Stock Repurchased During Period, Shares	0	0
Class B Common Stock
Statement of Stockholders' Equity [Abstract]
Stock Repurchased and Retired During Period, Shares	0	0