Fair Value Measurements - Schedule of assets and liabilities that were accounted for at fair value (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
ASSETS
Fixed maturities, AFS, at fair value		$ 30,928,988	$ 30,256,602
Short-term investments, at fair value		815,481	625,193
Equity securities, AFS, at fair value		713,967	1,037,331
Total investments available for sale		32,458,436	31,919,126
Fixed maturities, trading, at fair value		2,006,385	1,617,014
Short-term investments, at fair value		14,965	9,563
Total investments trading		2,021,350	1,626,577
Collateral and counterparty netting, asset		(3,579)	(17,947)
Liabilities:
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable		999,219	998,968
Collateral and counterparty netting, liability		(3,579)	(17,947)
Recurring
ASSETS
Total investments available for sale		32,458,436	31,919,126
Cash equivalents	[1]	1,550,707	1,669,652
Other investments	[2]	792,639	699,402
Other assets	[3]	29,215	89,549
Total assets accounted for at fair value		36,920,329	36,077,104
Collateral and counterparty netting, asset	[3]	(3,579)	(17,947)
Liabilities:
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable	[4]	1,112,969	937,721
Other liabilities	[3]	59,317	46,757
Total liabilities accounted for at fair value		1,172,286	984,478
Collateral and counterparty netting, liability	[3]	(3,579)	(17,947)
Recurring | Quoted prices in active markets for identical assets (Level 1)
ASSETS
Total investments available for sale		557,510	898,664
Cash equivalents	[1]	279,224	573,398
Other investments	[2]	0	0
Other assets	[3]	0	0
Total assets accounted for at fair value		836,734	1,472,062
Liabilities:
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable	[4]	0	0
Other liabilities	[3]	0	0
Total liabilities accounted for at fair value		0	0
Recurring | Significant other observable inputs (Level 2)
ASSETS
Total investments available for sale		31,876,112	30,974,396
Cash equivalents	[1]	1,271,483	1,096,254
Other investments	[2]	570,931	493,874
Other assets	[3]	14,467	87,655
Total assets accounted for at fair value		35,822,325	34,351,554
Liabilities:
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable	[4]	1,112,969	937,721
Other liabilities	[3]	44,591	44,865
Total liabilities accounted for at fair value		1,157,560	982,586
Recurring | Significant other unobservable inputs (Level 3)
ASSETS
Total investments available for sale		24,814	46,066
Cash equivalents	[1]	0	0
Other investments	[2]	221,708	205,528
Other assets	[3]	18,327	19,841
Total assets accounted for at fair value		264,849	271,435
Liabilities:
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable	[4]	0	0
Other liabilities	[3]	18,305	19,839
Total liabilities accounted for at fair value		18,305	19,839
Exclude Life Funds Withheld Assets
ASSETS
Fixed maturities, AFS, at fair value		28,901,876	27,948,560
Short-term investments, at fair value		815,481	625,193
Equity securities, AFS, at fair value		713,967	1,037,331
Total investments available for sale		30,431,324	29,611,084
Exclude Life Funds Withheld Assets | U.S. Government and Government - Related/Supported
ASSETS
Fixed maturities, AFS, at fair value		4,362,740	3,894,388
Exclude Life Funds Withheld Assets | U.S. States and political subdivisions of the States
ASSETS
Fixed maturities, AFS, at fair value		2,064,640	2,478,112
Exclude Life Funds Withheld Assets | Non-U.S. Governments
ASSETS
Fixed maturities, AFS, at fair value		5,201,581	5,030,132
Exclude Life Funds Withheld Assets | Corporate
ASSETS
Fixed maturities, AFS, at fair value		9,795,133	10,134,729
Exclude Life Funds Withheld Assets | RMBS
ASSETS
Fixed maturities, AFS, at fair value		4,724,402	4,492,625
Exclude Life Funds Withheld Assets | CMBS
ASSETS
Fixed maturities, AFS, at fair value		1,244,017	665,186
Exclude Life Funds Withheld Assets | Other asset-backed securities
ASSETS
Fixed maturities, AFS, at fair value		1,509,363	1,253,388
Exclude Life Funds Withheld Assets | Recurring
ASSETS
Fixed maturities, AFS, at fair value		28,901,876	27,948,560
Short-term investments, at fair value	[5]	815,481	625,193
Equity securities, AFS, at fair value		713,967	1,037,331
Total investments available for sale		30,431,324	29,611,084
Exclude Life Funds Withheld Assets | Recurring | U.S. Government and Government - Related/Supported
ASSETS
Fixed maturities, AFS, at fair value		4,362,740	3,894,388
Exclude Life Funds Withheld Assets | Recurring | U.S. States and political subdivisions of the States
ASSETS
Fixed maturities, AFS, at fair value		2,064,640	2,478,112
Exclude Life Funds Withheld Assets | Recurring | Non-U.S. Governments
ASSETS
Fixed maturities, AFS, at fair value		5,201,581	5,030,132
Exclude Life Funds Withheld Assets | Recurring | Corporate
ASSETS
Fixed maturities, AFS, at fair value		9,795,133	10,134,729
Exclude Life Funds Withheld Assets | Recurring | RMBS
ASSETS
Fixed maturities, AFS, at fair value		4,724,402	4,492,625
Exclude Life Funds Withheld Assets | Recurring | CMBS
ASSETS
Fixed maturities, AFS, at fair value		1,244,017	665,186
Exclude Life Funds Withheld Assets | Recurring | Other asset-backed securities
ASSETS
Fixed maturities, AFS, at fair value		1,509,363	1,253,388
Exclude Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1)
ASSETS
Fixed maturities, AFS, at fair value		0	0
Short-term investments, at fair value	[5]	0	0
Equity securities, AFS, at fair value		557,510	898,664
Total investments available for sale		557,510	898,664
Exclude 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, AFS, at fair value		0	0
Exclude Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | U.S. States and political subdivisions of the States
ASSETS
Fixed maturities, AFS, at fair value		0	0
Exclude Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | Non-U.S. Governments
ASSETS
Fixed maturities, AFS, at fair value		0	0
Exclude Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | Corporate
ASSETS
Fixed maturities, AFS, at fair value		0	0
Exclude Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | RMBS
ASSETS
Fixed maturities, AFS, at fair value		0	0
Exclude Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | CMBS
ASSETS
Fixed maturities, AFS, at fair value		0	0
Exclude Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | Other asset-backed securities
ASSETS
Fixed maturities, AFS, at fair value		0	0
Exclude Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2)
ASSETS
Fixed maturities, AFS, at fair value		28,877,062	27,902,494
Short-term investments, at fair value	[5]	815,481	625,193
Equity securities, AFS, at fair value		156,457	138,667
Total investments available for sale		29,849,000	28,666,354
Exclude Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | U.S. Government and Government - Related/Supported
ASSETS
Fixed maturities, AFS, at fair value		4,344,450	3,869,637
Exclude Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | U.S. States and political subdivisions of the States
ASSETS
Fixed maturities, AFS, at fair value		2,064,640	2,478,112
Exclude Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Non-U.S. Governments
ASSETS
Fixed maturities, AFS, at fair value		5,201,581	5,030,132
Exclude Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Corporate
ASSETS
Fixed maturities, AFS, at fair value		9,793,841	10,114,644
Exclude Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | RMBS
ASSETS
Fixed maturities, AFS, at fair value		4,724,402	4,492,625
Exclude Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | CMBS
ASSETS
Fixed maturities, AFS, at fair value		1,244,017	665,087
Exclude Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Other asset-backed securities
ASSETS
Fixed maturities, AFS, at fair value		1,504,131	1,252,257
Exclude Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3)
ASSETS
Fixed maturities, AFS, at fair value		24,814	46,066
Short-term investments, at fair value	[5]	0	0
Equity securities, AFS, at fair value		0	0
Total investments available for sale		24,814	46,066
Exclude Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | U.S. Government and Government - Related/Supported
ASSETS
Fixed maturities, AFS, at fair value		18,290	24,751
Exclude Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | U.S. States and political subdivisions of the States
ASSETS
Fixed maturities, AFS, at fair value		0	0
Exclude Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Non-U.S. Governments
ASSETS
Fixed maturities, AFS, at fair value		0	0
Exclude Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Corporate
ASSETS
Fixed maturities, AFS, at fair value		1,292	20,085
Exclude Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | RMBS
ASSETS
Fixed maturities, AFS, at fair value		0	0
Exclude Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | CMBS
ASSETS
Fixed maturities, AFS, at fair value		0	99
Exclude Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Other asset-backed securities
ASSETS
Fixed maturities, AFS, at fair value		5,232	1,131
Life Funds Withheld Assets
ASSETS
Fixed maturities, AFS, at fair value		2,027,112	2,308,042
Fixed maturities, trading, at fair value		2,006,385	1,617,014
Short-term investments, at fair value		14,965	9,563
Total investments trading		2,021,350	1,626,577
Life Funds Withheld Assets | U.S. Government and Government - Related/Supported
ASSETS
Fixed maturities, AFS, at fair value		11,316	10,577
Fixed maturities, trading, at fair value		11,042	14,708
Life Funds Withheld Assets | U.S. States and political subdivisions of the States
ASSETS
Fixed maturities, trading, at fair value		31	224
Life Funds Withheld Assets | Non-U.S. Governments
ASSETS
Fixed maturities, AFS, at fair value		584,534	624,677
Fixed maturities, trading, at fair value		485,171	444,944
Life Funds Withheld Assets | Corporate
ASSETS
Fixed maturities, AFS, at fair value		1,137,213	1,454,608
Fixed maturities, trading, at fair value		1,460,292	1,111,205
Life Funds Withheld Assets | RMBS
ASSETS
Fixed maturities, AFS, at fair value		515	22,380
Fixed maturities, trading, at fair value		932	961
Life Funds Withheld Assets | CMBS
ASSETS
Fixed maturities, AFS, at fair value		122,272	94,240
Fixed maturities, trading, at fair value		7,580	5,526
Life Funds Withheld Assets | Other asset-backed securities
ASSETS
Fixed maturities, AFS, at fair value		171,262	101,560
Fixed maturities, trading, at fair value		41,337	39,446
Life Funds Withheld Assets | Recurring
ASSETS
Fixed maturities, AFS, at fair value		2,027,112	2,308,042
Fixed maturities, trading, at fair value		2,006,385	1,617,014
Short-term investments, at fair value	[5]	14,965	9,563
Total investments trading		2,021,350	1,626,577
Cash equivalents	[1]	67,982	72,798
Life Funds Withheld Assets | Recurring | U.S. Government and Government - Related/Supported
ASSETS
Fixed maturities, AFS, at fair value		11,316	10,577
Fixed maturities, trading, at fair value		11,042	14,708
Life Funds Withheld Assets | Recurring | U.S. States and political subdivisions of the States
ASSETS
Fixed maturities, trading, at fair value		31	224
Life Funds Withheld Assets | Recurring | Non-U.S. Governments
ASSETS
Fixed maturities, AFS, at fair value		584,534	624,677
Fixed maturities, trading, at fair value		485,171	444,944
Life Funds Withheld Assets | Recurring | Corporate
ASSETS
Fixed maturities, AFS, at fair value		1,137,213	1,454,608
Fixed maturities, trading, at fair value		1,460,292	1,111,205
Life Funds Withheld Assets | Recurring | RMBS
ASSETS
Fixed maturities, AFS, at fair value		515	22,380
Fixed maturities, trading, at fair value		932	961
Life Funds Withheld Assets | Recurring | CMBS
ASSETS
Fixed maturities, AFS, at fair value		122,272	94,240
Fixed maturities, trading, at fair value		7,580	5,526
Life Funds Withheld Assets | Recurring | Other asset-backed securities
ASSETS
Fixed maturities, AFS, at fair value		171,262	101,560
Fixed maturities, trading, at fair value		41,337	39,446
Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1)
ASSETS
Fixed maturities, AFS, at fair value		0	0
Fixed maturities, trading, at fair value		0	0
Short-term investments, at fair value	[5]	0	0
Total investments trading		0	0
Cash equivalents	[1]	0	0
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, AFS, 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) | U.S. States and political subdivisions of the States
ASSETS
Fixed maturities, trading, at fair value		0	0
Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | Non-U.S. Governments
ASSETS
Fixed maturities, AFS, 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) | Corporate
ASSETS
Fixed maturities, AFS, 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
ASSETS
Fixed maturities, AFS, 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) | CMBS
ASSETS
Fixed maturities, AFS, 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) | Other asset-backed securities
ASSETS
Fixed maturities, AFS, at fair value		0	0
Fixed maturities, trading, at fair value		0	0
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2)
ASSETS
Fixed maturities, AFS, at fair value		2,027,112	2,308,042
Fixed maturities, trading, at fair value		2,006,385	1,617,014
Short-term investments, at fair value	[5]	14,965	9,563
Total investments trading		2,021,350	1,626,577
Cash equivalents	[1]	67,982	72,798
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | U.S. Government and Government - Related/Supported
ASSETS
Fixed maturities, AFS, at fair value		11,316	10,577
Fixed maturities, trading, at fair value		11,042	14,708
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | U.S. States and political subdivisions of the States
ASSETS
Fixed maturities, trading, at fair value		31	224
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Non-U.S. Governments
ASSETS
Fixed maturities, AFS, at fair value		584,534	624,677
Fixed maturities, trading, at fair value		485,171	444,944
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Corporate
ASSETS
Fixed maturities, AFS, at fair value		1,137,213	1,454,608
Fixed maturities, trading, at fair value		1,460,292	1,111,205
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | RMBS
ASSETS
Fixed maturities, AFS, at fair value		515	22,380
Fixed maturities, trading, at fair value		932	961
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | CMBS
ASSETS
Fixed maturities, AFS, at fair value		122,272	94,240
Fixed maturities, trading, at fair value		7,580	5,526
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Other asset-backed securities
ASSETS
Fixed maturities, AFS, at fair value		171,262	101,560
Fixed maturities, trading, at fair value		41,337	39,446
Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3)
ASSETS
Fixed maturities, AFS, at fair value		0	0
Fixed maturities, trading, at fair value		0	0
Short-term investments, at fair value	[5]	0	0
Total investments trading		0	0
Cash equivalents	[1]	0	0
Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | U.S. Government and Government - Related/Supported
ASSETS
Fixed maturities, AFS, at fair value		0	0
Fixed maturities, trading, at fair value		0	0
Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | U.S. States and political subdivisions of the States
ASSETS
Fixed maturities, trading, at fair value		0	0
Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Non-U.S. Governments
ASSETS
Fixed maturities, AFS, at fair value		0	0
Fixed maturities, trading, at fair value		0	0
Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Corporate
ASSETS
Fixed maturities, AFS, 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
ASSETS
Fixed maturities, AFS, at fair value		0	0
Fixed maturities, trading, at fair value		0	0
Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | CMBS
ASSETS
Fixed maturities, AFS, at fair value		0	0
Fixed maturities, trading, at fair value		0	0
Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Other asset-backed securities
ASSETS
Fixed maturities, AFS, at fair value		0	0
Fixed maturities, trading, at fair value		$ 0	$ 0

[1] 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.

[2] Excluded from Other Investments are certain investments that are measured using net asset value as a practical expedient in the amount of $198.2 million and $322.9 million as of December 31, 2017 and December 31, 2016, respectively. Under GAAP, these investments are excluded from the fair value hierarchy table. In addition, the Other investments balance excludes loans held at amortized cost, which totaled $173.0 million at December 31, 2017 and $142.3 million at December 31, 2016.

[3] Other assets and other liabilities include derivative instruments. The derivative balances included in each category are reported gross of cash collateral by level with a counterparty netting adjustment presented separately in the Counterparty Netting column. The fair values of the individual derivative contracts are reported gross in their respective levels based on the fair value hierarchy.

[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 GreyCastle Life Retro Arrangements described in Note 1(a), "Significant Accounting Policies - Basis of Preparation and Consolidation", accrue to the benefit of GCLR.

[5] Short-term investments consist primarily of U.S. and Non-U.S. Government securities, Corporate securities and Other asset-backed securities.