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Variable Interest Entities and Noncontrolling Interests - Variable interest entity (Details) - USD ($) $ in Thousands		1 Months Ended	9 Months Ended
	Jul. 02, 2019	Mar. 31, 2014	Sep. 30, 2020	Sep. 30, 2019
Variable Interest Entity [Line Items]
Net cash provided by (used for) operating activities			$ 2,331,450	$ 1,543,139
Net cash provided by (used for) investing activities			(2,608,418)	(1,250,476)
Net cash (used for) provided by financing activities			$ 566,090	(46,496)
Variable Interest Entity, Primary Beneficiary \| Watford Holdings Ltd
Variable Interest Entity [Line Items]
Initial investment contribution amount		$ 100,000
Number of shares acquired		2,500,000
Ownership percentage		11.00%	13.00%
Senior notes, face amount	$ 175,000
Senior notes, stated percentage	6.50%
Proceeds from issuance of senior notes	$ 172,400
Senior notes acquired	$ 35,000
Net cash provided by (used for) operating activities			$ 133,600	177,200
Net cash provided by (used for) investing activities			242,000	(181,200)
Net cash (used for) provided by financing activities			$ (279,700)	$ 22,200