Variable Interest Entities and Noncontrolling Interests - Variable interest entity (Details) - USD ($) $ in Thousands	1 Months Ended	6 Months Ended
	Mar. 31, 2014	Jun. 30, 2018	Jun. 30, 2017
Variable Interest Entity [Line Items]
Net cash provided by operating activities		$ 496,886	$ 533,535
Net cash used for investing activities		102,399	(289,115)
Net cash used for financing activities		(733,029)	(327,790)
Variable Interest Entity, Primary Beneficiary | Watford Holdings Ltd
Variable Interest Entity [Line Items]
Initial investment contribution amount	$ 100,000
Ownership percentage	11.00%
Variable Interest Entity, Primary Beneficiary | Watford Re
Variable Interest Entity [Line Items]
Net cash provided by operating activities		92,400	134,400
Net cash used for investing activities		(168,000)	(63,500)
Net cash used for financing activities		$ (2,100)	$ (76,400)