Statutory Information - Summary of statutory capital, surplus and net income (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Bermuda
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	$ 6,447,803	[1]	$ 5,416,948	[1]
Required statutory capital and surplus	2,209,757	[1]	2,077,442	[1]
Statutory net income (loss)	692,676		719,267		631,483
Ireland
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	553,561	[1]	533,283	[1]
Required statutory capital and surplus	463,877	[1]	458,666	[1]
Statutory net income (loss)	43,197		(24,410)		(1,940)
United States
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	1,454,282	[1]	1,013,228	[1]
Required statutory capital and surplus	467,797	[1]	335,442	[1]
Statutory net income (loss)	46,110		62,605		(21,517)
United Kingdom
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	384,525	[1]	392,734	[1]
Required statutory capital and surplus	307,596	[1]	353,330	[1]
Statutory net income (loss)	13,016		(11,353)		(4,449)
Canada
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	79,123	[1]	77,877	[1]
Required statutory capital and surplus	58,837	[1]	74,951	[1]
Statutory net income (loss)	$ 1,517		$ (36,203)		$ 0

[1] Such amounts include ownership interests in affiliated insurance and reinsurance subsidiaries.