Investments - Narrative (Details) $ in Thousands		12 Months Ended
	May 01, 2014 USD ($)	Dec. 31, 2016 USD ($) positions	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Marketable Securities [Line Items]
Cumulative adjustments recorded to value of future policy benefit reserves, amount		$ (62,295)	$ (170,688)	$ 400,456
Fixed maturities, AFS, at fair value		$ 30,256,602	$ 32,257,589
Percentage of fixed income portfolio invested in securities below investment grade		2.40%	2.00%
Amortized cost of fixed maturities, available for sale		$ 29,587,656	$ 31,517,654
Percentage of gross unrealized losses relating to securities below investment grade		4.10%	14.70%
Pledged assets, amount		$ 17,100,000	$ 18,300,000
Gross unrealized losses		$ 370,017	369,975
Total number of positions - AFS | positions		8,495
AFS securities in unrealized loss position (number of positions) | positions		3,037
OTTI on investments, net of amounts transferred to other comprehensive income		$ 78,600	83,000	35,700
Credit loss impairments previously recognized on securities which matured, paid down, prepaid or were sold during the period		18,178	54,648
Equity securities, AFS, at fair value		1,037,331	878,919
Cost of equity securities, available for sale		996,610	834,079
Fixed Income Funds Member
Marketable Securities [Line Items]
Cost of equity securities, available for sale		100,000
XL Re Life (SAC) Ltd [Member]
Marketable Securities [Line Items]
Held To Maturity Securities Transferred Security Carrying Value	$ 2,800,000
Fixed maturities, held to maturity, gross unrealized gains	424,900
Fixed maturities, held to maturity, gross unrealized losses	0
Unrealized Gains (Losses) on Investments
Marketable Securities [Line Items]
Cumulative adjustments recorded to value of future policy benefit reserves, amount		(62,300)
Unrealized Gains (Losses) on Investments | XL Re Life (SAC) Ltd [Member]
Marketable Securities [Line Items]
Cumulative adjustments recorded to value of future policy benefit reserves, amount	$ 440,500
Exclude Life Funds Withheld Assets
Marketable Securities [Line Items]
Fixed maturities, AFS, at fair value		27,948,560	29,104,486
Amortized cost of fixed maturities, available for sale		27,768,929	28,839,622
Gross unrealized losses		370,017	369,975
Equity securities, AFS, at fair value		1,037,331	878,919
Cost of equity securities, available for sale		996,610	834,079
Exclude Life Funds Withheld Assets | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Marketable Securities [Line Items]
Fixed maturities, AFS, at fair value		4,218,367	3,757,971
Amortized cost of fixed maturities, available for sale		4,213,845	3,697,756
Exclude Life Funds Withheld Assets | Equity securities
Marketable Securities [Line Items]
OTTI on investments, net of amounts transferred to other comprehensive income		30,126	15,954	10,944
Exclude Life Funds Withheld Assets | Structured Credit Securities [Member]
Marketable Securities [Line Items]
OTTI on investments, net of amounts transferred to other comprehensive income		3,000	1,300	4,300
Exclude Life Funds Withheld Assets | Foreign Exchange [Member]
Marketable Securities [Line Items]
OTTI on investments, net of amounts transferred to other comprehensive income		29,900	23,600	4,300
Exclude Life Funds Withheld Assets | Other Investments [Member]
Marketable Securities [Line Items]
OTTI on investments, net of amounts transferred to other comprehensive income		0	8,700	12,500
Exclude Life Funds Withheld Assets | RMBS - Non-Agency
Marketable Securities [Line Items]
Fixed maturities, AFS, at fair value		274,258	328,540
Amortized cost of fixed maturities, available for sale		265,877	319,876
Exclude Life Funds Withheld Assets | CMBS
Marketable Securities [Line Items]
Fixed maturities, AFS, at fair value		665,186	405,316
Amortized cost of fixed maturities, available for sale		670,005	401,713
Exclude Life Funds Withheld Assets | Collateralized debt obligations ('CDO')
Marketable Securities [Line Items]
Fixed maturities, AFS, at fair value		377,491	32,410
Amortized cost of fixed maturities, available for sale		379,532	41,679
No Longer Intend To Hold For A Period Sufficient To Recover Fair Value To Cost [Member] | Exclude Life Funds Withheld Assets
Marketable Securities [Line Items]
OTTI on investments, net of amounts transferred to other comprehensive income		29,300	29,400	3,700
In A Loss Position For More Than 11 Months Or Greater Than Fifty Percent Of Amortized Cost [Member] | Exclude Life Funds Withheld Assets
Marketable Securities [Line Items]
OTTI on investments, net of amounts transferred to other comprehensive income		$ 16,400	$ 20,000	$ 10,900