Capital Stock (Details) - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Capital Unit [Line Items]
Treasury stock, shares (shares)	24,055,870		24,344,813
Common stock, shares authorized (shares)	180,000,000
Common stock, par value (dollars per share)	$ 0		$ 0
Common stock, shares, outstanding (shares)	38,025,521		37,736,578
4 1/2% Cumulative Preferred Nonvoting Stock
Capital Unit [Line Items]
Preferred stock, shares authorized (shares)	100,000
Preferred stock, par value (dollars per share)	$ 50
Percentage of cumulative preferred stock (percent)	4.50%
Preferred stock, callable price per share (dollars per share)	$ 52.50
Preferred stock, shares issued (shares)	71,340
Treasury stock, shares (shares)	52,878
Purchase of treasury shares (shares)	0
Second Cumulative Preferred Voting Stock
Capital Unit [Line Items]
Preferred stock, shares authorized (shares)	1,000,000
Preferred stock, par value (dollars per share)	$ 10
Preferred stock, shares issued (shares)	0
Common Stock
Capital Unit [Line Items]
Purchase of treasury shares (shares)	168,941	0
Common stock, shares issued (shares)			62,081,391
Stock issued during period, new issues (shares)	0
Common stock, value, issued			$ 100,000,000
Treasury Stock
Capital Unit [Line Items]
Reissued shares (shares)	526,890