Segment Information Underwriting Margins and Combined Ratios for our Underwriting Operations (Detail) - Underwriting operations	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Personal Lines | Agency
Segment Reporting Information [Line Items]
Underwriting Margin	4.90%	14.00%	9.10%	14.90%
Combined Ratio	95.10%	86.00%	90.90%	85.10%
Personal Lines | Direct
Segment Reporting Information [Line Items]
Underwriting Margin	2.80%	15.50%	6.00%	13.70%
Combined Ratio	97.20%	84.50%	94.00%	86.30%
Operating Segments
Segment Reporting Information [Line Items]
Underwriting Margin	3.50%	12.30%	7.00%	12.70%
Combined Ratio	96.50%	87.70%	93.00%	87.30%
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Underwriting Margin	3.80%	14.80%	7.50%	14.30%
Combined Ratio	96.20%	85.20%	92.50%	85.70%
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Underwriting Margin	8.00%	15.90%	11.80%	12.60%
Combined Ratio	92.00%	84.10%	88.20%	87.40%
Operating Segments | Property
Segment Reporting Information [Line Items]
Underwriting Margin	(16.60%)	(43.60%)	(15.80%)	(16.30%)
Combined Ratio	116.60%	143.60%	115.80%	116.30%
Combined ratio amortization points	0.028	0.033	0.029	0.034