Earnings Per Share and Capital Stock (Details) - USD ($)		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic
Net Income		$ 12,868,000	$ 21,006,000	$ 22,792,000	$ 12,701,000	$ 11,212,000	$ 21,639,000	$ 16,742,000	$ 10,150,000	$ 69,367,000	$ 59,743,000	$ 60,484,000
Less: Distributed earnings to common shareholders										36,417,000	33,408,000	32,690,000
Less: Distributed earnings to participating securities										197,000	187,000	207,000
Undistributed earnings										32,753,000	26,148,000	27,587,000
Undistributed earnings allocated to common shareholders										32,577,000	26,003,000	27,414,000
Undistributed earnings allocated to participating securities											145,000	173,000
Total income available to common shareholders, basic										$ 68,994,000	$ 59,411,000	$ 60,104,000
Weighted average Common Shares outstanding, basic										36,638,000	36,552,000	37,389,000
Basic earnings per Common Share:
Income from continuing operations (in dollars per share)		$ 0.35	$ 0.57	$ 0.62	$ 0.35	$ 0.30	$ 0.59	$ 0.46	$ 0.28	$ 1.88	$ 1.63	$ 1.61
Net Income (in dollars per share)										$ 1.88	$ 1.63	$ 1.61
Diluted
Total income available to common shareholders, basic										$ 68,994,000	$ 59,411,000	$ 60,104,000
Undistributed earnings for dilutive stock options										176,000	145,000	173,000
Total common shareholders earnings, diluted										$ 69,170,000	$ 59,556,000	$ 60,277,000
Weighted average Common Shares outstanding, basic										36,638,000	36,552,000	37,389,000
Stock-based compensation (in shares)	[1]									206,000	198,000	225,000
Weighted average common shares outstanding, diluted										36,844,000	36,750,000	37,614,000
Diluted earnings per Common Share:
Income from continuing operations (in dollars per share)		$ 0.35	$ 0.57	$ 0.62	$ 0.34	$ 0.30	$ 0.59	$ 0.45	$ 0.28	$ 1.88	$ 1.62	$ 1.60
Net Income (in dollars per share)										$ 1.88	$ 1.62	$ 1.60
Stock option and restricted stock option outstanding
Options outstanding (in shares)		69,202								69,202
Share based compensation arrangement
Proceeds from Stock Options Exercised, Distributed to Subsidiaries										$ 0	$ 0	$ 0
Dividend Reinvestment Plan Common Stock Capital Shares Reserved for Future Issuance		1,055,948								1,055,948
Common Shares authorized for issuance but unissued under 401(k) Plan		387,300								387,300
Restricted Stock Units
Share based compensation arrangement
Restricted stock units outstanding (in shares)		205,166								205,166

[1] (1) In applying the treasury stock method of reflecting the dilutive effect of outstanding stock-based compensation in the calculation of diluted EPS, 69,202 stock options and 205,166 restricted stock units, including performance awards, at December 31, 2017 were deemed to be outstanding in accordance with accounting guidance on earnings per share.