Reinsurance - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Future Policy Benefit, by Product Segment [Line Items]
Reinsurance recoverable from GCLR	$ 3,072,615	$ 3,719,131
Reinsurance recoveries on losses and loss expenses recorded during period	1,700,000	1,600,000	$ 700,000
Total reinsurance assets	6,100,000	5,700,000
Reinsurance balances receivable	577,479	418,666
Unpaid losses and loss expenses recoverable	5,491,297	5,262,706
Total allowance for uncollectible reinsurance	62,800	82,100
Collateral held related to reinsurance recoverables	4,100,000	3,800,000
Total allowance recorded relating to reinsurance recoverables	42,119	58,200
Total allowance recorded relating to reinsurance balances receivable	20,700	23,900
U.S. Term Life Reinsurance [Member]
Liability for Future Policy Benefit, by Product Segment [Line Items]
Reinsurance recoverable from GCLR	270,238	284,481
XL Re Life (SAC) Ltd [Member]
Liability for Future Policy Benefit, by Product Segment [Line Items]
Reinsurance recoverable from GCLR	$ 3,072,615	$ 3,719,131