Segment Information - Narrative (Details) - segment	12 Months Ended
	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018
Segment Reporting Information [Line Items]
Number of reportable segments	5
Number of operating segments	5
Syndicate 1729
Segment Reporting Information [Line Items]
Proportion of capital provided to support Lloyd's syndicate (percent)		61.00%	62.00%
Workers' Compensation Insurance
Segment Reporting Information [Line Items]
Alternative market solutions percentage ceded	100.00%
Syndicate 6131
Segment Reporting Information [Line Items]
Proportion of capital provided to support Lloyd's syndicate (percent)				100.00%