Fair Value Measurements - Schedule of assets and liabilities that were accounted for at fair value (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Assets
Fixed maturities, at fair value	$ 29,110,400		$ 29,359,034
Equity securities, at fair value	783,471		868,292
Short-term investments, at fair value	168,862		256,727
Total investments available for sale	30,062,733		30,484,053
Fixed maturities, trading at fair value	139,377		1,171
Collateral and counterparty netting, assets	(587)	[1]	(696)	[1]
Liabilities
Funds withheld on life retrocession arrangements (net of future policy benefit reserves recoverable)	1,138,652		1,155,016
Collateral and counterparty nettings, liabilities	(587)	[1]	(696)	[1]
Recurring
Assets
Total investments available for sale	30,062,733		30,484,053
Cash equivalents	2,228,498	[2]	1,501,832
Other investments	974,583	[3]	894,057	[3]
Other assets	257,293	[1]	135,963	[1]
Total assets accounted for at fair value	33,783,993		33,150,274
Liabilities
Funds withheld on life retrocession arrangements (net of future policy benefit reserves recoverable)	654,561	[4]	450,831	[4]
Financial instruments sold, but not yet purchased	7,232	[5]	30,406	[5]
Other liabilities	121,648	[1]	30,488	[1]
Total liabilities accounted for at fair value	783,441		511,725
Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Total investments available for sale	497,496		502,284
Cash equivalents	1,835,344	[2]	1,103,877
Other investments	0	[3]	0	[3]
Other assets	0	[1]	0	[1]
Total assets accounted for at fair value	2,333,628		1,606,621
Liabilities
Funds withheld on life retrocession arrangements (net of future policy benefit reserves recoverable)	0	[4]	0	[4]
Financial instruments sold, but not yet purchased	1,572	[5]	4,737	[5]
Other liabilities	0	[1]	0	[1]
Total liabilities accounted for at fair value	1,572		4,737
Recurring | Significant other observable inputs (Level 2)
Assets
Total investments available for sale	29,058,389		29,280,719
Cash equivalents	393,154	[2]	397,955
Other investments	784,486	[3]	708,974	[3]
Other assets	244,900	[1]	122,996	[1]
Total assets accounted for at fair value	30,741,027		30,644,553
Liabilities
Funds withheld on life retrocession arrangements (net of future policy benefit reserves recoverable)	654,561	[4]	450,831	[4]
Financial instruments sold, but not yet purchased	5,660	[5]	25,669	[5]
Other liabilities	99,649	[1]	7,757	[1]
Total liabilities accounted for at fair value	759,870		484,257
Recurring | Significant other unobservable inputs (Level 3)
Assets
Total investments available for sale	506,848		701,050
Cash equivalents	0	[2]	0
Other investments	190,097	[3]	185,083	[3]
Other assets	12,980	[1]	13,663	[1]
Total assets accounted for at fair value	709,925		899,796
Liabilities
Funds withheld on life retrocession arrangements (net of future policy benefit reserves recoverable)	0	[4]	0	[4]
Financial instruments sold, but not yet purchased	0	[5]	0	[5]
Other liabilities	22,586	[1]	23,427	[1]
Total liabilities accounted for at fair value	22,586		23,427
Excluding Life Funds Withheld Assets
Assets
Fixed maturities, at fair value	24,392,421		24,177,469
Equity securities, at fair value	783,471		868,292
Short-term investments, at fair value	168,862		256,727
Total investments available for sale	25,344,754		25,302,488
Excluding Life Funds Withheld Assets | U.S. Government and Government - Related/Supported
Assets
Fixed maturities, at fair value	3,378,342		2,171,953
Excluding Life Funds Withheld Assets | Corporate
Assets
Fixed maturities, at fair value	8,154,638	[6]	8,778,373	[7]
Excluding Life Funds Withheld Assets | RMBS - Agency
Assets
Fixed maturities, at fair value	3,711,334		3,728,576
Excluding Life Funds Withheld Assets | RMBS - Non-Agency
Assets
Fixed maturities, at fair value	380,135		427,351
Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value	1,032,720		1,052,544
Excluding Life Funds Withheld Assets | Collateralized debt obligations
Assets
Fixed maturities, at fair value	500,974		692,034
Excluding Life Funds Withheld Assets | Other asset-backed securities
Assets
Fixed maturities, at fair value	1,167,367		1,065,293
Excluding Life Funds Withheld Assets | US States and political subdivisions of the States
Assets
Fixed maturities, at fair value	1,973,140		2,021,272
Excluding Life Funds Withheld Assets | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Assets
Fixed maturities, at fair value	4,093,771		4,240,073
Excluding Life Funds Withheld Assets | Recurring
Assets
Fixed maturities, at fair value	24,392,421		24,177,469
Equity securities, at fair value	783,471		868,292
Short-term investments, at fair value	168,862	[8],[9]	256,727	[8],[9]
Total investments available for sale	25,344,754		25,302,488
Excluding Life Funds Withheld Assets | Recurring | U.S. Government and Government - Related/Supported
Assets
Fixed maturities, at fair value	3,378,342		2,171,953
Excluding Life Funds Withheld Assets | Recurring | Corporate
Assets
Fixed maturities, at fair value	8,154,638	[8]	8,778,373	[8]
Excluding Life Funds Withheld Assets | Recurring | RMBS - Agency
Assets
Fixed maturities, at fair value	3,711,334		3,728,576
Excluding Life Funds Withheld Assets | Recurring | RMBS - Non-Agency
Assets
Fixed maturities, at fair value	380,135		427,351
Excluding Life Funds Withheld Assets | Recurring | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value	1,032,720		1,052,544
Excluding Life Funds Withheld Assets | Recurring | Collateralized debt obligations
Assets
Fixed maturities, at fair value	500,974		692,034
Excluding Life Funds Withheld Assets | Recurring | Other asset-backed securities
Assets
Fixed maturities, at fair value	1,167,367		1,065,293
Excluding Life Funds Withheld Assets | Recurring | US States and political subdivisions of the States
Assets
Fixed maturities, at fair value	1,973,140		2,021,272
Excluding Life Funds Withheld Assets | Recurring | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Assets
Fixed maturities, at fair value	4,093,771		4,240,073
Excluding Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed maturities, at fair value	0		0
Equity securities, at fair value	497,496		502,284
Short-term investments, at fair value	0	[8],[9]	0	[8],[9]
Total investments available for sale	497,496		502,284
Excluding Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | U.S. Government and Government - Related/Supported
Assets
Fixed maturities, at fair value	0		0
Excluding Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | Corporate
Assets
Fixed maturities, at fair value	0	[8]	0	[8]
Excluding Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | RMBS - Agency
Assets
Fixed maturities, at fair value	0		0
Excluding Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | RMBS - Non-Agency
Assets
Fixed maturities, at fair value	0		0
Excluding Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value	0		0
Excluding Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | Collateralized debt obligations
Assets
Fixed maturities, at fair value	0		0
Excluding Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | Other asset-backed securities
Assets
Fixed maturities, at fair value	0		0
Excluding Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | US States and political subdivisions of the States
Assets
Fixed maturities, at fair value	0		0
Excluding Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Assets
Fixed maturities, at fair value	0		0
Excluding Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2)
Assets
Fixed maturities, at fair value	23,885,573		23,476,419
Equity securities, at fair value	285,975		366,008
Short-term investments, at fair value	168,862	[8],[9]	256,727	[8],[9]
Total investments available for sale	24,340,410		24,099,154
Excluding Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | U.S. Government and Government - Related/Supported
Assets
Fixed maturities, at fair value	3,378,342		2,171,953
Excluding Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Corporate
Assets
Fixed maturities, at fair value	8,148,777	[8]	8,772,479	[8]
Excluding Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | RMBS - Agency
Assets
Fixed maturities, at fair value	3,709,514		3,726,666
Excluding Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | RMBS - Non-Agency
Assets
Fixed maturities, at fair value	380,135		427,351
Excluding Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value	1,032,720		1,052,544
Excluding Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Collateralized debt obligations
Assets
Fixed maturities, at fair value	4,051		4,076
Excluding Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Other asset-backed securities
Assets
Fixed maturities, at fair value	1,165,123		1,060,005
Excluding Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | US States and political subdivisions of the States
Assets
Fixed maturities, at fair value	1,973,140		2,021,272
Excluding Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Assets
Fixed maturities, at fair value	4,093,771		4,240,073
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3)
Assets
Fixed maturities, at fair value	506,848		701,050
Equity securities, at fair value	0		0
Short-term investments, at fair value	0	[8],[9]	0	[8],[9]
Total investments available for sale	506,848		701,050
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | U.S. Government and Government - Related/Supported
Assets
Fixed maturities, at fair value	0		0
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Corporate
Assets
Fixed maturities, at fair value	5,861	[8]	5,894	[8]
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | RMBS - Agency
Assets
Fixed maturities, at fair value	1,820		1,910
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | RMBS - Non-Agency
Assets
Fixed maturities, at fair value	0		0
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value	0		0
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Collateralized debt obligations
Assets
Fixed maturities, at fair value	496,923		687,958
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Other asset-backed securities
Assets
Fixed maturities, at fair value	2,244		5,288
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | US States and political subdivisions of the States
Assets
Fixed maturities, at fair value	0		0
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Assets
Fixed maturities, at fair value	0		0
Life Funds Withheld Assets
Assets
Fixed maturities, at fair value	4,717,979		5,181,565
Fixed maturities, trading at fair value	139,377		1,171
Life Funds Withheld Assets | U.S. Government and Government - Related/Supported
Assets
Fixed maturities, at fair value	17,613		18,724
Life Funds Withheld Assets | Corporate
Assets
Fixed maturities, at fair value	2,746,226	[6]	2,817,980	[7]
Fixed maturities, trading at fair value	137,438		1,171
Life Funds Withheld Assets | RMBS - Agency
Assets
Fixed maturities, at fair value	2,130		3,782
Life Funds Withheld Assets | RMBS - Non-Agency
Assets
Fixed maturities, at fair value	38,808		85,335
Life Funds Withheld Assets | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value	159,257		193,167
Life Funds Withheld Assets | Other asset-backed securities
Assets
Fixed maturities, at fair value	244,140		273,541
Fixed maturities, trading at fair value	1,939
Life Funds Withheld Assets | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Assets
Fixed maturities, at fair value	1,509,805		1,789,036
Life Funds Withheld Assets | Recurring
Assets
Fixed maturities, at fair value	4,717,979		5,181,565
Fixed maturities, trading at fair value	139,377		1,171
Cash equivalents	121,509	[2]	133,198	[2]
Life Funds Withheld Assets | Recurring | U.S. Government and Government - Related/Supported
Assets
Fixed maturities, at fair value	17,613		18,724
Life Funds Withheld Assets | Recurring | Corporate
Assets
Fixed maturities, at fair value	2,746,226		2,817,980
Fixed maturities, trading at fair value	137,438		1,171
Life Funds Withheld Assets | Recurring | RMBS - Agency
Assets
Fixed maturities, at fair value	2,130		3,782
Life Funds Withheld Assets | Recurring | RMBS - Non-Agency
Assets
Fixed maturities, at fair value	38,808		85,335
Life Funds Withheld Assets | Recurring | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value	159,257		193,167
Life Funds Withheld Assets | Recurring | Other asset-backed securities
Assets
Fixed maturities, at fair value	244,140		273,541
Fixed maturities, trading at fair value	1,939
Life Funds Withheld Assets | Recurring | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Assets
Fixed maturities, at fair value	1,509,805		1,789,036
Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed maturities, at fair value	0		0
Fixed maturities, trading at fair value	0		0
Cash equivalents	788	[2]	460	[2]
Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | U.S. Government and Government - Related/Supported
Assets
Fixed maturities, at fair value	0		0
Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | Corporate
Assets
Fixed maturities, at fair value	0		0
Fixed maturities, trading at fair value	0		0
Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | RMBS - Agency
Assets
Fixed maturities, at fair value	0		0
Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | RMBS - Non-Agency
Assets
Fixed maturities, at fair value	0		0
Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value	0		0
Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | Other asset-backed securities
Assets
Fixed maturities, at fair value	0		0
Fixed maturities, trading at fair value	0
Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Assets
Fixed maturities, at fair value	0		0
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2)
Assets
Fixed maturities, at fair value	4,717,979		5,181,565
Fixed maturities, trading at fair value	139,377		1,171
Cash equivalents	120,721	[2]	132,738	[2]
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | U.S. Government and Government - Related/Supported
Assets
Fixed maturities, at fair value	17,613		18,724
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Corporate
Assets
Fixed maturities, at fair value	2,746,226		2,817,980
Fixed maturities, trading at fair value	137,438		1,171
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | RMBS - Agency
Assets
Fixed maturities, at fair value	2,130		3,782
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | RMBS - Non-Agency
Assets
Fixed maturities, at fair value	38,808		85,335
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value	159,257		193,167
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Other asset-backed securities
Assets
Fixed maturities, at fair value	244,140		273,541
Fixed maturities, trading at fair value	1,939
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Assets
Fixed maturities, at fair value	1,509,805		1,789,036
Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3)
Assets
Fixed maturities, at fair value	0		0
Fixed maturities, trading at fair value	0		0
Cash equivalents	0	[2]	0	[2]
Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | U.S. Government and Government - Related/Supported
Assets
Fixed maturities, at fair value	0		0
Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Corporate
Assets
Fixed maturities, at fair value	0		0
Fixed maturities, trading at fair value	0		0
Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | RMBS - Agency
Assets
Fixed maturities, at fair value	0		0
Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | RMBS - Non-Agency
Assets
Fixed maturities, at fair value	0		0
Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value	0		0
Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Other asset-backed securities
Assets
Fixed maturities, at fair value	0		0
Fixed maturities, trading at fair value	0
Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Assets
Fixed maturities, at fair value	$ 0		$ 0

[1] Other assets and other liabilities include derivative instruments. The derivative balances included in each category are reported on a gross basis by level with a netting adjustment presented separately in the Collateral and Counterparty Netting column. The fair values of the individual derivative contracts are reported gross in their respective levels based on the fair value hierarchy. For further details regarding derivative fair values and associated collateral received or paid see Note 7, “Derivative Instruments,” to the Unaudited Consolidated Financial Statements.

[2] Cash equivalents balances subject to fair value measurement include certificates of deposit and money market funds. Operating cash balances are not subject to recurring fair value measurement guidance.

[3] The Other investments balance excludes certain structured transactions including certain investments in project finance transactions, a payment obligation and liquidity financing provided to a structured credit vehicle as a part of a third party medium term note facility. These investments, which totaled $354.4 million as of March 31, 2015 and $354.4 million at December 31, 2014, are carried at amortized cost. For further information, see Item 8, Note 8, “Other Investments,” to the Consolidated Financial Statements included in our Annual Report on Form 10-K for the year ended December 31, 2014.

[4] Funds withheld on life retrocession arrangements (net of future policy benefit reserves recoverable) include balances related to the life retrocession embedded derivative, under which all investment results associated with the Life Funds Withheld Assets related to the Life Retro Arrangements described in Note 3(c), ”Acquisition and Disposals - Sale of Life Reinsurance Subsidiary," accrue to the benefit of GCLR.

[5] Financial instruments sold, but not yet purchased, represent “short sales” and are included within “Payable for investments purchased” on the balance sheets.

[6] Included are certain medium term notes supported primarily by pools of European investment grade credit with varying degrees of leverage. The notes have a fair value of $74.8 million and an amortized cost of $63.8 million. These notes allow the investor to participate in cash flows of the underlying bonds including certain residual values, which could serve to either decrease or increase the ultimate values of these notes.

[7] Included are certain medium term notes supported primarily by pools of European investment grade credit with varying degrees of leverage. The notes have a fair value of $79.9 million and an amortized cost of $68.4 million. These notes allow the investor to participate in cash flows of the underlying bonds including certain residual values, which could serve to either decrease or increase the ultimate values of these notes.

[8] Included are certain medium term notes supported primarily by pools of European investment grade credit with varying degrees of leverage. The notes had a fair value of $74.8 million and $79.9 million and an amortized cost of $63.8 million and $68.4 million as of March 31, 2015 and December 31, 2014, respectively. These notes allow the investor to participate in cash flows of the underlying bonds including certain residual values, which could serve to either decrease or increase the ultimate values of these notes.

[9] Short-term investments consist primarily of Corporate securities and U.S. and Non-U.S. Government and Government-Related/Supported securities.