Segment Information - Underwriting Margins and Combined Ratios for our Underwriting Operations (Detail) - Underwriting operations	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Personal Lines | Agency
Segment Reporting Information [Line Items]
Underwriting Margin	6.00%	4.90%	6.30%	9.10%
Combined Ratio	94.00%	95.10%	93.70%	90.90%
Personal Lines | Direct
Segment Reporting Information [Line Items]
Underwriting Margin	4.00%	2.80%	3.60%	6.00%
Combined Ratio	96.00%	97.20%	96.40%	94.00%
Operating Segments
Segment Reporting Information [Line Items]
Underwriting Margin	4.40%	3.50%	5.00%	7.00%
Combined Ratio	95.60%	96.50%	95.00%	93.00%
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Underwriting Margin	4.90%	3.80%	4.90%	7.50%
Combined Ratio	95.10%	96.20%	95.10%	92.50%
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Underwriting Margin	10.50%	8.00%	10.10%	11.80%
Combined Ratio	89.50%	92.00%	89.90%	88.20%
Operating Segments | Property
Segment Reporting Information [Line Items]
Underwriting Margin	(27.50%)	(16.60%)	(13.20%)	(15.80%)
Combined Ratio	127.50%	116.60%	113.20%	115.80%
Combined ratio amortization points	0.009	0.028	0.017	0.029