Common Shares, Redeemable Common Shares and Preferred Shares Common shares outstanding (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) shares
Stock split, conversion ratio	2
Preferred shares
Preferred stock, shares authorized	4,000,000
Preferred stock, par value | $ / shares	$ 0
Preferred stock, shares issued	0
Redeemable stock, outstanding	5,642,154	6,467,027	7,056,904
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 (in dollars per share) | $ / shares	$ 1.00	$ 1.00
Common stock, shares issued	37,271,606	36,446,890	35,856,856
Treasury stock, shares	20,033,108	18,692,901	17,990,620
Common shares, direct sales	17,992
Common shares, value, direct sales | $	$ 345
Stock issued during period, shares, employee benefit plan	395,586
Stock issued during period, value, employee retirement plan | $	$ 7,647
Common shares, employer matching contribution, shares	198,189
Employee retirement plan, employer match | $	$ 3,785
Accrued 401K match | $	$ 4,126
Stock issued during period, shares, employee stock purchase plan	348,973
Increase (decrease) in stockholders' equity [Roll Forward]
Shares outstanding, beginning of year	24,220,859	24,923,140
Shares purchased	(2,636,943)	(1,697,727)
Shares sold	960,740	662,806
Stock subscription offering, employee cash purchases	129,245	90,645
Options exercised	206,751	241,995
Shares outstanding, end of year	22,880,652	24,220,859	24,923,140
Per share amounts and common shares outstanding [Abstract]
Share-based compensation arrangement by share-based payment award, options, exercisable, number	889,814
Stock subscription offering, number of partially-paid subscriptions	690,260
Stock subscription offering, number of rights outstanding	241,236
Treasury stock [Member]
Common stock, number of shares, par value and other disclosures [Abstract]
Shares purchased, value | $	$ 51,116	$ 30,566	$ 25,290
Increase (decrease) in stockholders' equity [Roll Forward]
Shares purchased	(2,637,000)	(1,698,000)	(1,492,000)
Shares sold	961,000	663,000	617,000