Segment Information Underwriting Margins and Combined Ratios for our Underwriting Operations (Detail) - Underwriting operations	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Personal Lines | Agency
Segment Reporting Information [Line Items]
Underwriting Margin	13.40%	15.70%
Combined Ratio	86.60%	84.30%
Personal Lines | Direct
Segment Reporting Information [Line Items]
Underwriting Margin	9.40%	11.90%
Combined Ratio	90.60%	88.10%
Operating Segments
Segment Reporting Information [Line Items]
Underwriting Margin	10.70%	13.10%
Combined Ratio	89.30%	86.90%
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Underwriting Margin	11.30%	13.70%
Combined Ratio	88.70%	86.30%
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Underwriting Margin	16.10%	9.50%
Combined Ratio	83.90%	90.50%
Operating Segments | Property
Segment Reporting Information [Line Items]
Underwriting Margin	(15.00%)	11.70%
Combined Ratio	115.00%	88.30%
Combined ratio amortization points	0.030	0.034