Fair Value Measurements - Schedule of assets and liabilities that were accounted for at fair value (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Assets
Fixed maturities AFS		$ 29,793,151	$ 30,928,988
Short-term investments, at fair value		623,113	815,481
Total investments available for sale		30,416,264	32,458,436
Fixed maturities, trading at fair value		1,990,344	2,006,385
Short-term investments, trading at fair value		5,788	14,965
Equity securities, at fair value		644,711	713,967
Counterparty netting, assets		(7,627)	(3,579)
Liabilities
Funds withheld on GreyCastle life retrocession arrangements		736,092	999,219
Counterparty netting, liabilities		(7,627)	(3,579)
Excluding Life Funds Withheld Assets
Assets
Fixed maturities AFS		28,201,417	28,901,876
Short-term investments, at fair value		623,113	815,481
Equity securities, at fair value			713,967
Total investments available for sale		28,824,530	30,431,324
Excluding Life Funds Withheld Assets | U.S. Government and Government Agencies
Assets
Fixed maturities AFS		3,960,291	4,362,740
Excluding Life Funds Withheld Assets | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities AFS		1,858,142	2,064,640
Excluding Life Funds Withheld Assets | Non-U.S. Governments
Assets
Fixed maturities AFS		5,115,860	5,201,581
Excluding Life Funds Withheld Assets | Corporate
Assets
Fixed maturities AFS		9,950,095	9,795,133
Excluding Life Funds Withheld Assets | Residential mortgage-backed securities
Assets
Fixed maturities AFS		4,254,036	4,724,402
Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Assets
Fixed maturities AFS		1,363,850	1,244,017
Excluding Life Funds Withheld Assets | Other asset-backed securities
Assets
Fixed maturities AFS		1,699,143	1,509,363
Life Funds Withheld Assets
Assets
Fixed maturities AFS		1,591,734	2,027,112
Fixed maturities, trading at fair value		1,990,344	2,006,385
Short-term investments, trading at fair value		5,788	14,965
Total investments trading		1,996,132	2,021,350
Life Funds Withheld Assets | U.S. Government and Government Agencies
Assets
Fixed maturities AFS		10,934	11,316
Fixed maturities, trading at fair value		9,633	11,042
Life Funds Withheld Assets | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, trading at fair value			31
Life Funds Withheld Assets | Non-U.S. Governments
Assets
Fixed maturities AFS		467,920	584,534
Fixed maturities, trading at fair value		489,276	485,171
Life Funds Withheld Assets | Corporate
Assets
Fixed maturities AFS		869,594	1,137,213
Fixed maturities, trading at fair value		1,440,572	1,460,292
Life Funds Withheld Assets | Residential mortgage-backed securities
Assets
Fixed maturities AFS		483	515
Fixed maturities, trading at fair value		919	932
Life Funds Withheld Assets | Commercial mortgage-backed securities
Assets
Fixed maturities AFS		119,611	122,272
Fixed maturities, trading at fair value		7,411	7,580
Life Funds Withheld Assets | Other asset-backed securities
Assets
Fixed maturities AFS		123,192	171,262
Fixed maturities, trading at fair value		42,533	41,337
Recurring
Assets
Total investments available for sale		30,416,264	32,458,436
Cash equivalents	[1]	1,460,940	1,550,707
Other investments	[2]	751,312	792,639
Other assets	[3]	74,917	29,215
Total assets accounted for at fair value		35,409,161	36,920,329
Counterparty netting, assets	[3]	(7,627)	(3,579)
Liabilities
Funds withheld on GreyCastle life retrocession arrangements	[4]	889,813	1,112,969
Other liabilities	[3]	40,024	59,317
Total liabilities accounted for at fair value		929,837	1,172,286
Counterparty netting, liabilities	[3]	(7,627)	(3,579)
Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Total investments available for sale		0	557,510
Cash equivalents	[1]	380,587	279,224
Other investments	[2]	0	0
Other assets	[3]	0	0
Total assets accounted for at fair value		873,059	836,734
Liabilities
Funds withheld on GreyCastle life retrocession arrangements	[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		30,403,250	31,876,112
Cash equivalents	[1]	1,080,353	1,271,483
Other investments	[2]	533,090	570,931
Other assets	[3]	64,665	14,467
Total assets accounted for at fair value		34,294,614	35,822,325
Liabilities
Funds withheld on GreyCastle life retrocession arrangements	[4]	889,813	1,112,969
Other liabilities	[3]	29,829	44,591
Total liabilities accounted for at fair value		919,642	1,157,560
Recurring | Significant other unobservable inputs (Level 3)
Assets
Total investments available for sale		13,014	24,814
Cash equivalents	[1]	0	0
Other investments	[2]	218,222	221,708
Other assets	[3]	17,879	18,327
Total assets accounted for at fair value		249,115	264,849
Liabilities
Funds withheld on GreyCastle life retrocession arrangements	[4]	0	0
Other liabilities	[3]	17,822	18,305
Total liabilities accounted for at fair value		17,822	18,305
Recurring | Excluding Life Funds Withheld Assets
Assets
Fixed maturities AFS		28,201,417	28,901,876
Short-term investments, at fair value	[5]	623,113	815,481
Equity securities, at fair value			713,967
Total investments available for sale		28,824,530	30,431,324
Equity securities, at fair value		644,711
Recurring | Excluding Life Funds Withheld Assets | U.S. Government and Government Agencies
Assets
Fixed maturities AFS		3,960,291	4,362,740
Recurring | Excluding Life Funds Withheld Assets | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities AFS		1,858,142	2,064,640
Recurring | Excluding Life Funds Withheld Assets | Non-U.S. Governments
Assets
Fixed maturities AFS		5,115,860	5,201,581
Recurring | Excluding Life Funds Withheld Assets | Corporate
Assets
Fixed maturities AFS		9,950,095	9,795,133
Recurring | Excluding Life Funds Withheld Assets | Residential mortgage-backed securities
Assets
Fixed maturities AFS		4,254,036	4,724,402
Recurring | Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Assets
Fixed maturities AFS		1,363,850	1,244,017
Recurring | Excluding Life Funds Withheld Assets | Other asset-backed securities
Assets
Fixed maturities AFS		1,699,143	1,509,363
Recurring | Excluding Life Funds Withheld Assets | Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed maturities AFS		0	0
Short-term investments, at fair value	[5]	0	0
Equity securities, at fair value			557,510
Total investments available for sale		0	557,510
Equity securities, at fair value		492,472
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 AFS		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 AFS		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 AFS		0	0
Recurring | Excluding Life Funds Withheld Assets | Quoted prices in active markets for identical assets (Level 1) | Corporate
Assets
Fixed maturities AFS		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 AFS		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 AFS		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 AFS		0	0
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2)
Assets
Fixed maturities AFS		28,188,403	28,877,062
Short-term investments, at fair value	[5]	623,113	815,481
Equity securities, at fair value			156,457
Total investments available for sale		28,811,516	29,849,000
Equity securities, at fair value		152,239
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | U.S. Government and Government Agencies
Assets
Fixed maturities AFS		3,955,271	4,344,450
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities AFS		1,858,142	2,064,640
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Non-U.S. Governments
Assets
Fixed maturities AFS		5,115,860	5,201,581
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Corporate
Assets
Fixed maturities AFS		9,948,048	9,793,841
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities AFS		4,248,089	4,724,402
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities AFS		1,363,850	1,244,017
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Other asset-backed securities
Assets
Fixed maturities AFS		1,699,143	1,504,131
Recurring | Excluding Life Funds Withheld Assets | Significant other unobservable inputs (Level 3)
Assets
Fixed maturities AFS		13,014	24,814
Short-term investments, at fair value	[5]	0	0
Equity securities, at fair value			0
Total investments available for sale		13,014	24,814
Equity securities, at fair value		0
Recurring | Excluding Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | U.S. Government and Government Agencies
Assets
Fixed maturities AFS		5,020	18,290
Recurring | Excluding Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities AFS		0	0
Recurring | Excluding Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | Non-U.S. Governments
Assets
Fixed maturities AFS		0	0
Recurring | Excluding Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | Corporate
Assets
Fixed maturities AFS		2,047	1,292
Recurring | Excluding Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | Residential mortgage-backed securities
Assets
Fixed maturities AFS		5,947	0
Recurring | Excluding Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | Commercial mortgage-backed securities
Assets
Fixed maturities AFS		0	0
Recurring | Excluding Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | Other asset-backed securities
Assets
Fixed maturities AFS		0	5,232
Recurring | Life Funds Withheld Assets
Assets
Fixed maturities AFS		1,591,734	2,027,112
Fixed maturities, trading at fair value		1,990,344	2,006,385
Short-term investments, trading at fair value	[5]	5,788	14,965
Total investments trading		1,996,132	2,021,350
Cash equivalents	[1]	64,885	67,982
Recurring | Life Funds Withheld Assets | U.S. Government and Government Agencies
Assets
Fixed maturities AFS		10,934	11,316
Fixed maturities, trading at fair value		9,633	11,042
Recurring | Life Funds Withheld Assets | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, trading at fair value			31
Recurring | Life Funds Withheld Assets | Non-U.S. Governments
Assets
Fixed maturities AFS		467,920	584,534
Fixed maturities, trading at fair value		489,276	485,171
Recurring | Life Funds Withheld Assets | Corporate
Assets
Fixed maturities AFS		869,594	1,137,213
Fixed maturities, trading at fair value		1,440,572	1,460,292
Recurring | Life Funds Withheld Assets | Residential mortgage-backed securities
Assets
Fixed maturities AFS		483	515
Fixed maturities, trading at fair value		919	932
Recurring | Life Funds Withheld Assets | Commercial mortgage-backed securities
Assets
Fixed maturities AFS		119,611	122,272
Fixed maturities, trading at fair value		7,411	7,580
Recurring | Life Funds Withheld Assets | Other asset-backed securities
Assets
Fixed maturities AFS		123,192	171,262
Fixed maturities, trading at fair value		42,533	41,337
Recurring | Life Funds Withheld Assets | Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed maturities AFS		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 | Quoted prices in active markets for identical assets (Level 1) | U.S. Government and Government Agencies
Assets
Fixed maturities AFS		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 AFS		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 AFS		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 AFS		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 AFS		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 AFS		0	0
Fixed maturities, trading at fair value		0	0
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2)
Assets
Fixed maturities AFS		1,591,734	2,027,112
Fixed maturities, trading at fair value		1,990,344	2,006,385
Short-term investments, trading at fair value	[5]	5,788	14,965
Total investments trading		1,996,132	2,021,350
Cash equivalents	[1]	64,885	67,982
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2) | U.S. Government and Government Agencies
Assets
Fixed maturities AFS		10,934	11,316
Fixed maturities, trading at fair value		9,633	11,042
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			31
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Non-U.S. Governments
Assets
Fixed maturities AFS		467,920	584,534
Fixed maturities, trading at fair value		489,276	485,171
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Corporate
Assets
Fixed maturities AFS		869,594	1,137,213
Fixed maturities, trading at fair value		1,440,572	1,460,292
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities AFS		483	515
Fixed maturities, trading at fair value		919	932
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities AFS		119,611	122,272
Fixed maturities, trading at fair value		7,411	7,580
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Other asset-backed securities
Assets
Fixed maturities AFS		123,192	171,262
Fixed maturities, trading at fair value		42,533	41,337
Recurring | Life Funds Withheld Assets | Significant other unobservable inputs (Level 3)
Assets
Fixed maturities AFS		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 AFS		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 AFS		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 AFS		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 AFS		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 AFS		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 AFS		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 $227.2 million and $198.2 million at June 30, 2018 and December 31, 2017, 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 $173.2 million at June 30, 2018 and $173.0 million at December 31, 2017. 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, 2017.

[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(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 and Corporate securities.