Common Shares and Preferred Shares Common shares outstanding (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
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	24,000,000	24,000,000
Common stock, par value	$ 1.00	$ 1.00
Common stock, shares issued	21,456,880	21,457,000
Treasury stock, shares	7,730,878	7,611,245	7,344,624
Common shares, direct sales	8,549,000
Common shares, value, direct sales	$ 169,371,000
Stock issued during period, shares, employee retirement plan	56,430,000
Stock issued during period, value, employee retirement plan	1,133,000,000
Common shares, employer matching contribution, shares	125,518,000
Employee Retirement Plan, Employer Match	2,643,000	2,473,000	2,293,000
Stock issued during period, shares, employee stock purchase plan	136,947,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares outstanding, beginning of year	13,845,635	14,112,256
Shares purchased	(891,704)	(779,838)
Shares sold	327,442	351,285
Stock subscription offering, employee cash purchases	76,525	0
Options exercised	368,104	161,932
Increase/(decrease) in shares outstanding	(119,633)	(266,621)
Shares outstanding, end of year	13,726,002	13,845,635	14,112,256
Per Share Amounts and Common Shares Outstanding [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number	639,654
Stock subscription offering, number of partially-paid subscriptions	561,189
Stock subscription offering, number of rights outstanding	211,580
Treasury Stock [Member]
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares purchased, value	$ 18,103,000	$ 14,222,000	$ 20,711,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares purchased	(892,000)	(779,000)	(1,249,000)
Shares sold	(327,000)	(351,000)	(363,000)