Segment Information Underwriting Margins and Combined Ratios for our Underwriting Operations (Detail) - Underwriting operations	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Personal Lines | Agency
Segment Reporting Information [Line Items]
Underwriting Margin	11.20%	7.50%	12.20%	8.50%
Combined Ratio	88.80%	92.50%	87.80%	91.50%
Personal Lines | Direct
Segment Reporting Information [Line Items]
Underwriting Margin	9.00%	6.10%	9.40%	6.50%
Combined Ratio	91.00%	93.90%	90.60%	93.50%
Operating Segments
Segment Reporting Information [Line Items]
Underwriting Margin	9.10%	6.80%	10.30%	7.50%
Combined Ratio	90.90%	93.20%	89.70%	92.50%
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Underwriting Margin	10.10%	6.80%	10.80%	7.50%
Combined Ratio	89.90%	93.20%	89.20%	92.50%
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Underwriting Margin	11.30%	8.40%	11.50%	9.40%
Combined Ratio	88.70%	91.60%	88.50%	90.60%
Operating Segments | Property
Segment Reporting Information [Line Items]
Underwriting Margin	(16.60%)	1.50%	(3.90%)	2.50%
Combined Ratio	116.60%	98.50%	103.90%	97.50%
Combined ratio amortization points	0.058	0.065	0.060	0.067