Segment Information - Amortization of Other Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Amortization of other intangible assets	$ 14	$ 4	$ 4
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Amortization of other intangible assets	1	0	0
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Amortization of other intangible assets	4	4	4
Operating Segments | Group Benefits
Segment Reporting Information [Line Items]
Amortization of other intangible assets	$ 9	$ 0	$ 0