Segment Information (Income Before and After Federal Income Tax) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Net investment income	$ 50,618	$ 43,231
Net realized and unrealized investment gains (losses)	13,451	(10,549)
Total investment income, before federal income tax	64,069	32,682
Tax on investment income	12,120	5,226
Total investment income, after federal income tax	51,949	27,456
Standard Commercial Lines [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	25,815	6,804
Underwriting gain (loss), after federal income tax	$ 20,394	$ 5,375
Combined ratio	94.80%	98.50%
ROE Contribution	4.40%	1.30%
Standard Personal Lines [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	$ 3,167	$ (1,506)
Underwriting gain (loss), after federal income tax	$ 2,502	$ (1,190)
Combined ratio	95.90%	102.00%
ROE Contribution	0.50%	(0.30%)
Excess And Surplus Operations [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	$ 4,579	$ (565)
Underwriting gain (loss), after federal income tax	$ 3,617	$ (446)
Combined ratio	92.10%	101.10%
ROE Contribution	0.80%	(0.10%)
Investments [Member]
Segment Reporting Information [Line Items]
ROE Contribution	8.90%	8.50%

The after-tax income divided by average stockholders' equity.

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