Related Party Transactions - Narrative (Details) - management_company	Dec. 31, 2017	Dec. 31, 2016	Jul. 24, 2015
Related Party Transaction [Line Items]
Number of investment management companies	3	4
Number of investment technology firms	1	1
XL Innovate Fund [Member]
Related Party Transaction [Line Items]
Noncontrolling interest, ownership percentage by noncontrolling owners	5.21%		5.21%