Variable Interest Entity and Noncontrolling Interests - Variable interest entity (Details) (USD $) Share data in Millions, unless otherwise specified	1 Months Ended	12 Months Ended			1 Months Ended
	Oct. 30, 2001	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2014	Dec. 31, 2011
Variable Interest Entity [Line Items]
Proceeds from from common shares issued, net	$ 763,200,000
Proceeds on issuance of preference shares		0	0	315,763,000
Unearned premiums		2,231,578,000	1,896,365,000
Reserve for losses and loss adjustment expenses		9,036,448,000	8,824,696,000	8,933,292,000		8,456,210,000
Net cash provided by (used in) operating activities		1,037,130,000	850,868,000	921,603,000
Net cash provided by (used in) financing activities		539,943,000	524,018,000	(513,035,000)
Net cash provided by (used in) investing activities		1,505,421,000	1,307,513,000	395,968,000
Variable Interest Entity, Primary Beneficiary | Watford Re
Variable Interest Entity [Line Items]
Initial capitalization amount					1,100,000,000
Proceeds from from common shares issued, net					907,300,000
Net proceeds from issuance of common shares					895,600,000
Proceeds on issuance of preference shares					226,600,000
Net proceeds on issuance of preference shares					219,200,000
Initial investment contribution amount					100,000,000
Ownership percentage					11.00%
Number of warrants received in capital raising					1.7
Unearned premiums		180,000,000
Reserve for losses and loss adjustment expenses		66,300,000
Net cash provided by (used in) operating activities		39,300,000
Net cash provided by (used in) financing activities		1,160,000,000
Net cash provided by (used in) investing activities		$ 1,180,000,000