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, 2019	Sep. 30, 2018
Variable Interest Entity [Line Items]
Net cash used for operating activities			$ 1,543,139	$ 1,120,987
Net cash used for investing activities			(1,250,476)	(390,083)
Net cash provided by financing activities			(46,496)	(736,744)
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%
Number of shares called by warrants		975,503
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 used for operating activities			177,200	174,300
Net cash used for investing activities			(181,200)	(208,300)
Net cash provided by financing activities			$ 22,200	$ (24,200)