Losses and Loss Expenses - Reserving Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) classes	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Claims Development [Line Items]
Loss exposure classes | classes	250
Loss and loss expense reserves acquired in business combination | $	$ 101,315	$ 5,439,876	$ 0
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