Investments - Analysis of continual unrealized loss position (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
HTM securities
Less than 12 months Fair Value	$ 0	$ 46,034
Less than 12 month Gross Unrealized Losses	0	(941)
Equal to or greater than 12 months Fair Value	0	12,536
Equal to or greater than 12 months Gross Unrealized Losses	0	(669)
Fixed Maturities And Short Term Investments Available For Sale [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value		8,871,868
Less than 12 month Gross Unrealized Losses		(230,109)
Equal to or greater than 12 months Fair Value		2,117,444
Equal to or greater than 12 months Gross Unrealized Losses		(189,457)
U.S. Government and Government - Related/Supported [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value		1,333,704
Less than 12 month Gross Unrealized Losses		(30,474)
Equal to or greater than 12 months Fair Value		44,158
Equal to or greater than 12 months Gross Unrealized Losses		(3,614)
Corporate
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value		2,756,235
Less than 12 month Gross Unrealized Losses		(59,497)
Equal to or greater than 12 months Fair Value		513,106
Equal to or greater than 12 months Gross Unrealized Losses		(50,160)
HTM securities
Less than 12 months Fair Value		46,034
Less than 12 month Gross Unrealized Losses		(941)
Equal to or greater than 12 months Fair Value		642
Equal to or greater than 12 months Gross Unrealized Losses		(27)
RMBS - Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value		1,485,261
Less than 12 month Gross Unrealized Losses		(50,362)
Equal to or greater than 12 months Fair Value		169,704
Equal to or greater than 12 months Gross Unrealized Losses		(11,715)
RMBS - Non-Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value		14,204
Less than 12 month Gross Unrealized Losses		(604)
Equal to or greater than 12 months Fair Value		240,946
Equal to or greater than 12 months Gross Unrealized Losses		(30,522)
CMBS
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value		432,820
Less than 12 month Gross Unrealized Losses		(6,816)
Equal to or greater than 12 months Fair Value		107,192
Equal to or greater than 12 months Gross Unrealized Losses		(8,957)
Collateralized debt obligations ('CDO') [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value		58,239
Less than 12 month Gross Unrealized Losses		(217)
Equal to or greater than 12 months Fair Value		574,613
Equal to or greater than 12 months Gross Unrealized Losses		(42,717)
Other asset-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value		196,639
Less than 12 month Gross Unrealized Losses		(2,149)
Equal to or greater than 12 months Fair Value		96,528
Equal to or greater than 12 months Gross Unrealized Losses		(6,691)
U.S. States and political subdivisions of the States [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value		463,974
Less than 12 month Gross Unrealized Losses		(23,124)
Equal to or greater than 12 months Fair Value		64,324
Equal to or greater than 12 months Gross Unrealized Losses		(7,646)
Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value		2,130,792
Less than 12 month Gross Unrealized Losses		(56,866)
Equal to or greater than 12 months Fair Value		306,873
Equal to or greater than 12 months Gross Unrealized Losses		(27,435)
HTM securities
Less than 12 months Fair Value		0
Less than 12 month Gross Unrealized Losses		0
Equal to or greater than 12 months Fair Value		11,894
Equal to or greater than 12 months Gross Unrealized Losses		(642)
Equity securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	191,193	155,453
Less than 12 month Gross Unrealized Losses	(26,230)	(17,470)
Equal to or greater than 12 months Fair Value	0	0
Equal to or greater than 12 months Gross Unrealized Losses	0	0
Exclude Life Funds Withheld Assets [Member] \| Fixed Maturities And Short Term Investments Available For Sale [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	2,896,210
Less than 12 month Gross Unrealized Losses	(61,772)
Equal to or greater than 12 months Fair Value	2,745,705
Equal to or greater than 12 months Gross Unrealized Losses	(125,574)
Exclude Life Funds Withheld Assets [Member] \| U.S. Government and Government - Related/Supported [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	251,091
Less than 12 month Gross Unrealized Losses	(1,196)
Equal to or greater than 12 months Fair Value	342,890
Equal to or greater than 12 months Gross Unrealized Losses	(5,603)
Exclude Life Funds Withheld Assets [Member] \| Corporate
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	1,337,470
Less than 12 month Gross Unrealized Losses	(33,881)
Equal to or greater than 12 months Fair Value	424,221
Equal to or greater than 12 months Gross Unrealized Losses	(15,754)
Exclude Life Funds Withheld Assets [Member] \| RMBS - Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	134,535
Less than 12 month Gross Unrealized Losses	(220)
Equal to or greater than 12 months Fair Value	512,652
Equal to or greater than 12 months Gross Unrealized Losses	(10,563)
Exclude Life Funds Withheld Assets [Member] \| RMBS - Non-Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	45,378
Less than 12 month Gross Unrealized Losses	(1,358)
Equal to or greater than 12 months Fair Value	202,700
Equal to or greater than 12 months Gross Unrealized Losses	(16,797)
Exclude Life Funds Withheld Assets [Member] \| CMBS
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	78,356
Less than 12 month Gross Unrealized Losses	(385)
Equal to or greater than 12 months Fair Value	169,065
Equal to or greater than 12 months Gross Unrealized Losses	(4,877)
Exclude Life Funds Withheld Assets [Member] \| Collateralized debt obligations ('CDO') [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	249,803
Less than 12 month Gross Unrealized Losses	(2,666)
Equal to or greater than 12 months Fair Value	414,516
Equal to or greater than 12 months Gross Unrealized Losses	(24,503)
Exclude Life Funds Withheld Assets [Member] \| Other asset-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	143,044
Less than 12 month Gross Unrealized Losses	(2,813)
Equal to or greater than 12 months Fair Value	57,544
Equal to or greater than 12 months Gross Unrealized Losses	(3,232)
Exclude Life Funds Withheld Assets [Member] \| U.S. States and political subdivisions of the States [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	32,187
Less than 12 month Gross Unrealized Losses	(210)
Equal to or greater than 12 months Fair Value	63,695
Equal to or greater than 12 months Gross Unrealized Losses	(994)
Exclude Life Funds Withheld Assets [Member] \| Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	624,346
Less than 12 month Gross Unrealized Losses	(19,043)
Equal to or greater than 12 months Fair Value	558,422
Equal to or greater than 12 months Gross Unrealized Losses	$ (43,251)

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

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This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

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This item represents the fair value of securities categorized as held-to-maturity securities that have been in a continuous unrealized loss position for twelve months or longer.

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Amount of accumulated pre-tax unrealized loss on investments in held-to-maturity securities that have been in a loss position for twelve months or longer.

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This item represents the excess of amortized cost basis over fair value of securities that have been in a loss position for less than twelve months for those securities which are categorized as held-to-maturity.

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