Losses and Loss Expenses - Reserving Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Claims Development [Line Items]
Loss and loss expense reserves acquired in business combination	$ 0	$ 101,315	$ 5,439,876
Minimum
Claims Development [Line Items]
High layer excess casualty settlement period	5 years
Minimum | Property and other short-tail lines
Claims Development [Line Items]
Claims Reported Period	2 years
Minimum | Casualty and Other Long-Tail Lines
Claims Development [Line Items]
Claims Reported Period	3 years
Maximum
Claims Development [Line Items]
High layer excess casualty settlement period	15 years
Maximum | Property and other short-tail lines
Claims Development [Line Items]
Claims Reported Period	3 years
Maximum | Casualty and Other Long-Tail Lines
Claims Development [Line Items]
Claims Reported Period	20 years
Number of Reported Claims [Member] | Minimum
Claims Development [Line Items]
Immaterial Error Correction	0.03
Number of Reported Claims [Member] | Maximum
Claims Development [Line Items]
Immaterial Error Correction	0.13