Fair Value Measurements - Schedule of assets and liabilities that were accounted for at fair value (Details) - USD ($) $ in Thousands		Sep. 30, 2017	Dec. 31, 2016
Assets
Fixed maturities, at fair value		$ 31,675,509	$ 30,256,602
Short-term investments, at fair value		336,054	625,193
Equity securities, at fair value		1,011,942	1,037,331
Total investments available for sale		33,023,505	31,919,126
Fixed maturities, trading at fair value		1,866,465	1,617,014
Short-term investments, trading at fair value		9,361	9,563
Total investments trading		1,875,826	1,626,577
Collateral and counterparty netting, assets		(3,681)	(17,947)
Liabilities
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable: 2017 - $3,221,803; 2016 - $3,072,615)		920,571	998,968
Collateral and counterparty nettings, liabilities		(3,681)	(17,947)
Excluding Life Funds Withheld Assets
Assets
Fixed maturities, at fair value		29,545,375	27,948,560
Short-term investments, at fair value		336,054	625,193
Equity securities, at fair value		1,011,942	1,037,331
Total investments available for sale		30,893,371	29,611,084
Excluding Life Funds Withheld Assets | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		4,313,336	3,894,388
Excluding Life Funds Withheld Assets | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, at fair value		2,111,217	2,478,112
Excluding Life Funds Withheld Assets | Non-U.S. Governments
Assets
Fixed maturities, at fair value		5,260,637	5,030,132
Excluding Life Funds Withheld Assets | Corporate
Assets
Fixed maturities, at fair value		10,072,974	10,134,729
Excluding Life Funds Withheld Assets | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		4,941,702	4,492,625
Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		1,277,994	665,186
Excluding Life Funds Withheld Assets | Other asset-backed securities
Assets
Fixed maturities, at fair value		1,567,515	1,253,388
Life Funds Withheld Assets
Assets
Fixed maturities, at fair value		2,130,134	2,308,042
Fixed maturities, trading at fair value		1,866,465	1,617,014
Short-term investments, trading at fair value		9,361	9,563
Total investments trading		1,875,826	1,626,577
Life Funds Withheld Assets | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		11,090	10,577
Fixed maturities, trading at fair value		15,815	14,708
Life Funds Withheld Assets | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, trading at fair value			224
Life Funds Withheld Assets | Non-U.S. Governments
Assets
Fixed maturities, at fair value		611,843	624,677
Fixed maturities, trading at fair value		465,262	444,944
Life Funds Withheld Assets | Corporate
Assets
Fixed maturities, at fair value		1,211,551	1,454,608
Fixed maturities, trading at fair value		1,334,783	1,111,205
Life Funds Withheld Assets | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		589	22,380
Fixed maturities, trading at fair value		818	961
Life Funds Withheld Assets | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		122,223	94,240
Fixed maturities, trading at fair value		7,581	5,526
Life Funds Withheld Assets | Other asset-backed securities
Assets
Fixed maturities, at fair value		172,838	101,560
Fixed maturities, trading at fair value		42,206	39,446
Recurring
Assets
Total investments available for sale		33,023,505	31,919,126
Cash equivalents	[1]	1,629,519	1,669,652
Other investments	[2]	778,396	699,402
Other assets	[3]	19,563	89,549
Total assets accounted for at fair value		37,392,083	36,077,104
Collateral and counterparty netting, assets	[3]	(3,681)	(17,947)
Liabilities
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable: 2017 - $3,221,803; 2016 - $3,072,615)	[4]	1,037,270	937,721
Other liabilities	[3]	74,107	46,757
Total liabilities accounted for at fair value		1,111,377	984,478
Collateral and counterparty nettings, liabilities	[3]	(3,681)	(17,947)
Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Total investments available for sale		844,923	898,664
Cash equivalents	[1]	242,977	573,398
Other investments	[2]	0	0
Other assets	[3]	0	0
Total assets accounted for at fair value		1,088,405	1,472,062
Liabilities
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable: 2017 - $3,221,803; 2016 - $3,072,615)	[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		32,141,463	30,974,396
Cash equivalents	[1]	1,386,542	1,096,254
Other investments	[2]	558,934	493,874
Other assets	[3]	5,628	87,655
Total assets accounted for at fair value		36,033,162	34,351,554
Liabilities
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable: 2017 - $3,221,803; 2016 - $3,072,615)	[4]	1,037,270	937,721
Other liabilities	[3]	60,318	44,865
Total liabilities accounted for at fair value		1,097,588	982,586
Recurring | Significant other unobservable inputs (Level 3)
Assets
Total investments available for sale		37,119	46,066
Cash equivalents	[1]	0	0
Other investments	[2]	219,462	205,528
Other assets	[3]	17,616	19,841
Total assets accounted for at fair value		274,197	271,435
Liabilities
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable: 2017 - $3,221,803; 2016 - $3,072,615)	[4]	0	0
Other liabilities	[3]	17,470	19,839
Total liabilities accounted for at fair value		17,470	19,839
Recurring | Excluding Life Funds Withheld Assets
Assets
Fixed maturities, at fair value		29,545,375	27,948,560
Short-term investments, at fair value	[5]	336,054	625,193
Equity securities, at fair value		1,011,942	1,037,331
Total investments available for sale		30,893,371	29,611,084
Recurring | Excluding Life Funds Withheld Assets | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		4,313,336	3,894,388
Recurring | Excluding Life Funds Withheld Assets | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, at fair value		2,111,217	2,478,112
Recurring | Excluding Life Funds Withheld Assets | Non-U.S. Governments
Assets
Fixed maturities, at fair value		5,260,637	5,030,132
Recurring | Excluding Life Funds Withheld Assets | Corporate
Assets
Fixed maturities, at fair value		10,072,974	10,134,729
Recurring | Excluding Life Funds Withheld Assets | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		4,941,702	4,492,625
Recurring | Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		1,277,994	665,186
Recurring | Excluding Life Funds Withheld Assets | Other asset-backed securities
Assets
Fixed maturities, at fair value		1,567,515	1,253,388
Recurring | Excluding Life Funds Withheld Assets | Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed maturities, at fair value		0	0
Short-term investments, at fair value	[5]	0	0
Equity securities, at fair value		844,923	898,664
Total investments available for sale		844,923	898,664
Recurring | Excluding Life Funds Withheld Assets | Quoted prices in active markets for identical assets (Level 1) | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		0	0
Recurring | Excluding Life Funds Withheld Assets | Quoted prices in active markets for identical assets (Level 1) | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, at fair value		0	0
Recurring | Excluding Life Funds Withheld Assets | Quoted prices in active markets for identical assets (Level 1) | Non-U.S. Governments
Assets
Fixed maturities, at fair value		0	0
Recurring | Excluding Life Funds Withheld Assets | Quoted prices in active markets for identical assets (Level 1) | Corporate
Assets
Fixed maturities, at fair value		0	0
Recurring | Excluding Life Funds Withheld Assets | Quoted prices in active markets for identical assets (Level 1) | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		0	0
Recurring | Excluding Life Funds Withheld Assets | Quoted prices in active markets for identical assets (Level 1) | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		0	0
Recurring | Excluding Life Funds Withheld Assets | Quoted prices in active markets for identical assets (Level 1) | Other asset-backed securities
Assets
Fixed maturities, at fair value		0	0
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2)
Assets
Fixed maturities, at fair value		29,508,256	27,902,494
Short-term investments, at fair value	[5]	336,054	625,193
Equity securities, at fair value		167,019	138,667
Total investments available for sale		30,011,329	28,666,354
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		4,294,540	3,869,637
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, at fair value		2,111,217	2,478,112
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Non-U.S. Governments
Assets
Fixed maturities, at fair value		5,260,637	5,030,132
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Corporate
Assets
Fixed maturities, at fair value		10,071,680	10,114,644
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		4,941,702	4,492,625
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		1,277,216	665,087
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Other asset-backed securities
Assets
Fixed maturities, at fair value		1,551,264	1,252,257
Recurring | Excluding Life Funds Withheld Assets | Significant other unobservable inputs (Level 3)
Assets
Fixed maturities, at fair value		37,119	46,066
Short-term investments, at fair value	[5]	0	0
Equity securities, at fair value		0	0
Total investments available for sale		37,119	46,066
Recurring | Excluding Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		18,796	24,751
Recurring | Excluding Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, at fair value		0	0
Recurring | Excluding Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | Non-U.S. Governments
Assets
Fixed maturities, at fair value		0	0
Recurring | Excluding Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | Corporate
Assets
Fixed maturities, at fair value		1,294	20,085
Recurring | Excluding Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		0	0
Recurring | Excluding Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		778	99
Recurring | Excluding Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | Other asset-backed securities
Assets
Fixed maturities, at fair value		16,251	1,131
Recurring | Life Funds Withheld Assets
Assets
Fixed maturities, at fair value		2,130,134	2,308,042
Fixed maturities, trading at fair value		1,866,465	1,617,014
Short-term investments, trading at fair value	[5]	9,361	9,563
Total investments trading		1,875,826	1,626,577
Cash equivalents	[1]	65,274	72,798
Recurring | Life Funds Withheld Assets | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		11,090	10,577
Fixed maturities, trading at fair value		15,815	14,708
Recurring | Life Funds Withheld Assets | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, trading at fair value			224
Recurring | Life Funds Withheld Assets | Non-U.S. Governments
Assets
Fixed maturities, at fair value		611,843	624,677
Fixed maturities, trading at fair value		465,262	444,944
Recurring | Life Funds Withheld Assets | Corporate
Assets
Fixed maturities, at fair value		1,211,551	1,454,608
Fixed maturities, trading at fair value		1,334,783	1,111,205
Recurring | Life Funds Withheld Assets | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		589	22,380
Fixed maturities, trading at fair value		818	961
Recurring | Life Funds Withheld Assets | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		122,223	94,240
Fixed maturities, trading at fair value		7,581	5,526
Recurring | Life Funds Withheld Assets | Other asset-backed securities
Assets
Fixed maturities, at fair value		172,838	101,560
Fixed maturities, trading at fair value		42,206	39,446
Recurring | Life Funds Withheld Assets | 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
Short-term investments, trading at fair value	[5]	0	0
Total investments trading		0	0
Cash equivalents	[1]	505	0
Recurring | Life Funds Withheld Assets | Quoted prices in active markets for identical assets (Level 1) | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		0	0
Fixed maturities, trading at fair value		0	0
Recurring | Life Funds Withheld Assets | Quoted prices in active markets for identical assets (Level 1) | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, trading at fair value			0
Recurring | Life Funds Withheld Assets | Quoted prices in active markets for identical assets (Level 1) | Non-U.S. Governments
Assets
Fixed maturities, at fair value		0	0
Fixed maturities, trading at fair value		0	0
Recurring | Life Funds Withheld Assets | 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
Recurring | Life Funds Withheld Assets | Quoted prices in active markets for identical assets (Level 1) | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		0	0
Fixed maturities, trading at fair value		0	0
Recurring | Life Funds Withheld Assets | Quoted prices in active markets for identical assets (Level 1) | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		0	0
Fixed maturities, trading at fair value		0	0
Recurring | Life Funds Withheld Assets | 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	0
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2)
Assets
Fixed maturities, at fair value		2,130,134	2,308,042
Fixed maturities, trading at fair value		1,866,465	1,617,014
Short-term investments, trading at fair value	[5]	9,361	9,563
Total investments trading		1,875,826	1,626,577
Cash equivalents	[1]	64,769	72,798
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2) | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		11,090	10,577
Fixed maturities, trading at fair value		15,815	14,708
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2) | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, trading at fair value			224
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Non-U.S. Governments
Assets
Fixed maturities, at fair value		611,843	624,677
Fixed maturities, trading at fair value		465,262	444,944
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Corporate
Assets
Fixed maturities, at fair value		1,211,551	1,454,608
Fixed maturities, trading at fair value		1,334,783	1,111,205
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		589	22,380
Fixed maturities, trading at fair value		818	961
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		122,223	94,240
Fixed maturities, trading at fair value		7,581	5,526
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Other asset-backed securities
Assets
Fixed maturities, at fair value		172,838	101,560
Fixed maturities, trading at fair value		42,206	39,446
Recurring | Life Funds Withheld Assets | Significant other unobservable inputs (Level 3)
Assets
Fixed maturities, at fair value		0	0
Fixed maturities, trading at fair value		0	0
Short-term investments, trading at fair value	[5]	0	0
Total investments trading		0	0
Cash equivalents	[1]	0	0
Recurring | Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		0	0
Fixed maturities, trading at fair value		0	0
Recurring | Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, trading at fair value			0
Recurring | Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | Non-U.S. Governments
Assets
Fixed maturities, at fair value		0	0
Fixed maturities, trading at fair value		0	0
Recurring | Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | Corporate
Assets
Fixed maturities, at fair value		0	0
Fixed maturities, trading at fair value		0	0
Recurring | Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		0	0
Fixed maturities, trading at fair value		0	0
Recurring | Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		0	0
Fixed maturities, trading at fair value		0	0
Recurring | Life Funds Withheld Assets | 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	$ 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 $194.7 million and $322.9 million at September 30, 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. These investments, which totaled $172.5 million at September 30, 2017 and $142.3 million at December 31, 2016, are carried at amortized cost. For further information, see Item 8, Note 6, "Other Investments," to the Consolidated Financial Statements included in the Company's Annual Report on Form 10-K for the year ended 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. For further details regarding derivative fair values and associated collateral received or paid, see Note 5, "Derivative Instruments.

[4] Funds withheld on GreyCastle Life Retro 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, "Significant Accounting Policies," accrue to the benefit of GCLR.

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