Segment Information Underwriting Margins and Combined Ratios for our Underwriting Operations (Detail) - Underwriting operations	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Personal Lines | Agency
Segment Reporting Information [Line Items]
Underwriting Margin	10.30%	11.20%	11.70%	11.90%
Combined Ratio	89.70%	88.80%	88.30%	88.10%
Personal Lines | Direct
Segment Reporting Information [Line Items]
Underwriting Margin	7.70%	8.80%	8.50%	9.20%
Combined Ratio	92.30%	91.20%	91.50%	90.80%
Operating Segments
Segment Reporting Information [Line Items]
Underwriting Margin	8.10%	9.70%	9.60%	10.10%
Combined Ratio	91.90%	90.30%	90.40%	89.90%
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Underwriting Margin	9.00%	10.00%	10.10%	10.50%
Combined Ratio	91.00%	90.00%	89.90%	89.50%
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Underwriting Margin	6.30%	12.00%	11.30%	11.70%
Combined Ratio	93.70%	88.00%	88.70%	88.30%
Operating Segments | Property
Segment Reporting Information [Line Items]
Underwriting Margin	(2.30%)	(2.30%)	(3.10%)	(3.30%)
Combined Ratio	102.30%	102.30%	103.10%	103.30%
Combined ratio amortization points	0.040	0.054	0.045	0.058