Common Shares and Preferred Shares Common shares outstanding (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Stock subscription offering, number of shares subscribed	637,714
Preferred shares
Preferred stock, shares authorized	4,000,000
Preferred stock, par value	$ 0
Preferred stock, shares issued	0
Common stock, number of shares, par value and other disclosures [Abstract]
Common stock, shares authorized	48,000,000	48,000,000
Common stock, par value	$ 1.00	$ 1.00
Common stock, shares issued	21,456,880	21,456,880	21,456,880
Treasury stock, shares	8,995,310	8,713,566	8,291,947
Common shares, direct sales	12,121
Common shares, value, direct sales	$ 397
Stock issued during period, shares, employee benefit plan	44,986
Stock issued during period, value, employee retirement plan	$ 1,488
Common shares, employer matching contribution, shares	103,154
Employee retirement plan, employer match	$ 3,373
Defined contribution plan, cost recognized	$ 3,661	$ 3,373	$ 3,055
Stock issued during period, shares, employee stock purchase plan	148,631	125,779	115,029
Increase (decrease) in stockholders' equity [Roll Forward]
Shares outstanding, beginning of year	12,743,314	13,164,933
Shares purchased	(746,041)	(823,827)
Shares sold	308,892	287,875
Stock subscription offering, employee cash purchases	24,365	43,071
Options exercised	131,040	71,262
Increase/(decrease) in shares outstanding	(281,744)	(421,619)
Shares outstanding, end of year	12,461,570	12,743,314	13,164,933
Per share amounts and common shares outstanding [Abstract]
Share-based compensation arrangement by share-based payment award, options, exercisable, number	375,448
Stock subscription offering, number of partially-paid subscriptions	416,720
Stock subscription offering, number of rights outstanding	169,191
Treasury stock [Member]
Common stock, number of shares, par value and other disclosures [Abstract]
Shares purchased, value	$ 25,290	$ 25,395	$ 19,598
Increase (decrease) in stockholders' equity [Roll Forward]
Shares purchased	(746,000)	(824,000)	(727,000)
Shares sold	309,000	288,000	287,000