Accumulated Other Comprehensive Income, Retained Earnings (Details) - USD ($) $ in Thousands	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
Retained Earnings [Roll Forward]
Shareholders' equity at beginning of year:				$ 404,345				$ 394,498	$ 404,345	$ 394,498	$ 399,496
Net income	$ 16,476	$ 7,434	$ (12,343)	6,756	$ 4,864	$ 4,001	$ 5,969	14,112	18,323	28,945	23,283
Dividends									(16,302)	(15,803)	(15,003)
Repurchase of common shares									(1,880)	0	0
Shareholders' equity at end of year:	418,811				404,345				418,811	404,345	394,498
Retained Earnings [Member]
Retained Earnings [Roll Forward]
Shareholders' equity at beginning of year:				$ 316,195				$ 303,053	316,195	303,053	294,773
Net income									18,323	28,945	23,283
Dividends									(16,302)	(15,803)	(15,003)
Repurchase of common shares									(1,516)	0	0
Shareholders' equity at end of year:	$ 316,700				$ 316,195				$ 316,700	$ 316,195	$ 303,053