Consolidated Statements of Stockholders' Equity (USD $) In Thousands	Total	Common stock [Member]	Additional paid-in capital [Member]	Retained earnings [Member]	Accumulated other comprehensive income [Member]	Treasury stock [Member]
Beginning of year at Dec. 31, 2010 (As Previously Reported [Member])				$ 1,176,155
Beginning of year (Add: Adjustment for the cumulative effect on prior years of applying retroactively the new method of accounting for deferred policy acquisition costs [Member])				(53,068)
Beginning of year at Dec. 31, 2010		192,725	244,613	1,123,087	7,024	(549,408)
Dividend reinvestment plan (shares: 68,640 - 2012; 74,777 - 2011)		150	1,066
Stock purchase and compensation plans (shares: 698,723 - 2012; 607,059 - 2011)		1,214	8,260
Net income	3,999			3,999
Net income at Jan. 01, 2011 (As Previously Reported [Member])	3,732
Net income (Add: Adjustment for the cumulative effect on prior years of applying retroactively the new method of accounting for deferred policy acquisition costs [Member])	267
Dividends to stockholders ($0.39 per share - 2012 and 2011)				(21,579)
Other comprehensive income					25,140
Acquisition of treasury stock (shares: 176,513 - 2012; 137,667 - 2011)						(2,537)
End of period at Sep. 30, 2011	1,033,754	194,089	253,939	1,105,507	32,164	(551,945)
Beginning of year at Dec. 31, 2011 (As Previously Reported [Member])				1,116,319
Beginning of year (Add: Adjustment for the cumulative effect on prior years of applying retroactively the new method of accounting for deferred policy acquisition costs [Member])				0
Beginning of year at Dec. 31, 2011	1,058,328	194,494	257,370	1,116,319	42,294	(552,149)
Dividend reinvestment plan (shares: 68,640 - 2012; 74,777 - 2011)		137	1,064
Stock purchase and compensation plans (shares: 698,723 - 2012; 607,059 - 2011)		1,397	9,093
Net income	36,655			36,655
Dividends to stockholders ($0.39 per share - 2012 and 2011)				(21,859)
Other comprehensive income	42,717				42,717
Acquisition of treasury stock (shares: 176,513 - 2012; 137,667 - 2011)						(3,154)
End of period at Sep. 30, 2012	$ 1,124,378	$ 196,028	$ 267,527	$ 1,131,115	$ 85,011	$ (555,303)