CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Stockholders' Equity [Abstract]
Tax expense on change in retirement obligations, tax	$ 239	$ 2,107	$ 3,842
Tax from change on derivatives designated as cash flow hedges, tax	$ 2,445	$ 2,490	$ 2,499
Issuance of common stock from issuance of unvested shares, exercise of stock options and deferred compensation plans (in shares)	557,591	594,919	685,501
Issuance of common stock from issuance of unvested shares, exercise of stock options and deferred compensation plans, tax	$ 6,027	$ 5,305	$ 3,794
Repurchase of common stock (in shares)	266,000	738,593	2,811,002
Cash dividends declared, per common share outstanding (in dollars per share)	$ 1.48	$ 1.36	$ 1.28