Consolidated Statement of Earnings (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
REVENUE
Net earned premium	$ 601,832	$ 586,935	$ 572,248	$ 562,612	$ 560,030	$ 556,668	$ 561,356	$ 561,186	$ 2,323,627	$ 2,239,240	$ 2,242,625
Net investment income									221,620	220,182	222,634
Other operating income									40,932	35,452	30,448
Net realized investment gain									66,370	42,030	31,148
Other-than-temporary impairment credit losses											(1,028)
Total revenue	670,007	690,038	643,574	648,930	636,630	636,477	629,431	634,366	2,652,549	2,536,904	2,525,827
EXPENSE
Loss and loss adjustment expense, net	339,434	320,758	335,897	330,746	305,477	328,530	347,353	341,094	1,326,835	1,322,454	1,338,074
Policy acquisition costs, net									294,670	279,439	281,201
Other operating expense									341,083	336,091	326,497
Interest expense									28,125	26,210	25,628
Total expense	505,444	485,381	505,930	493,958	480,247	496,377	504,600	482,970	1,990,713	1,964,194	1,971,400
Earnings before income taxes	164,563	204,657	137,644	154,972	156,383	140,100	124,831	151,396	661,836	572,710	554,427
Income tax expense	51,534	64,390	40,508	47,061	41,373	41,925	36,669	45,546	203,493	165,513	163,187
Net earnings	$ 113,029	$ 140,267	$ 97,136	$ 107,911	$ 115,010	$ 98,175	$ 88,162	$ 105,850	$ 458,343	$ 407,197	$ 391,240
Earnings per common share
Basic	$ 1.17	$ 1.41	$ 0.97	$ 1.08	$ 1.15	$ 0.98	$ 0.88	$ 1.05	$ 4.62	$ 4.05	$ 3.84
Diluted	$ 1.16	$ 1.41	$ 0.97	$ 1.07	$ 1.14	$ 0.98	$ 0.87	$ 1.05	$ 4.61	$ 4.04	$ 3.83