CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock (in shares)	114,935	23,163	18,314
Issuance of common stock purchased under the Employee Stock Purchase Plan (in shares)	6,892	5,636	4,155
Forfeiture of common stock (in shares)	3,934	1,264	1,042
Release of shares of common stock under the employee stock ownership plan (in shares)	0	0	15,256
Cash dividends (in dollars per share)	$ 0.700	$ 0.650	$ 0.625
Purchase of common stock (in shares)	46,966	85,362	80,168