Segment Information (Income Before and After Federal Income Tax) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Net investment income	$ 69,716	$ 55,967
Net realized and unrealized investment gains (losses)	5,119	(44,666)
Total investment segment income, before federal income tax	74,835	11,301
Tax on investment income	14,448	1,104
Total investment segment income, after federal income tax	60,387	10,197
Standard Commercial Lines
Segment Reporting Information [Line Items]
Underwriting gain, before federal income tax	69,499	17,126
Underwriting gain, after federal income tax	$ 54,904	$ 13,529
Combined ratio	88.20%	96.70%
ROE Contribution	8.60%	2.40%
Standard Personal Lines
Segment Reporting Information [Line Items]
Underwriting gain, before federal income tax	$ 7,695	$ 387
Underwriting gain, after federal income tax	$ 6,079	$ 306
Combined ratio	89.60%	99.50%
ROE Contribution	1.00%	0.10%
E&S Lines
Segment Reporting Information [Line Items]
Underwriting gain, before federal income tax	$ 516	$ 3,844
Underwriting gain, after federal income tax	$ 408	$ 3,037
Combined ratio	99.20%	93.50%
ROE Contribution	0.10%	0.60%
Investments
Segment Reporting Information [Line Items]
ROE Contribution	8.90%	8.50%