Fair Value Measurements - Schedule of assets and liabilities that were accounted for at fair value (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Assets
Fixed maturities AFS		$ 30,656,628	$ 30,928,988
Short-term investments, at fair value		500,170	815,481
Total investments available for sale		31,156,798	32,458,436
Fixed maturities, trading at fair value		2,084,251	2,006,385
Short-term investments, trading at fair value		7,425	14,965
Equity securities, at fair value		637,872	713,967
Counterparty netting, assets		(7,239)	(3,579)
Liabilities
Funds withheld on GreyCastle life retrocession arrangements		989,140	999,219
Counterparty netting, liabilities		(7,239)	(3,579)
Excluding Life Funds Withheld Assets
Assets
Fixed maturities AFS		28,648,080	28,901,876
Short-term investments, at fair value		500,170	815,481
Equity securities, at fair value			713,967
Total investments available for sale		29,148,250	30,431,324
Excluding Life Funds Withheld Assets | U.S. Government and Government Agencies
Assets
Fixed maturities AFS		4,187,959	4,362,740
Excluding Life Funds Withheld Assets | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities AFS		2,013,162	2,064,640
Excluding Life Funds Withheld Assets | Non-U.S. Governments
Assets
Fixed maturities AFS		5,306,651	5,201,581
Excluding Life Funds Withheld Assets | Corporate
Assets
Fixed maturities AFS		9,742,670	9,795,133
Excluding Life Funds Withheld Assets | Residential mortgage-backed securities
Assets
Fixed maturities AFS		4,529,723	4,724,402
Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Assets
Fixed maturities AFS		1,259,519	1,244,017
Excluding Life Funds Withheld Assets | Other asset-backed securities
Assets
Fixed maturities AFS		1,608,396	1,509,363
Life Funds Withheld Assets
Assets
Fixed maturities AFS		2,008,548	2,027,112
Fixed maturities, trading at fair value		2,084,251	2,006,385
Short-term investments, trading at fair value		7,425	14,965
Total investments trading		2,091,676	2,021,350
Life Funds Withheld Assets | U.S. Government and Government Agencies
Assets
Fixed maturities AFS		11,616	11,316
Fixed maturities, trading at fair value		9,320	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		584,406	584,534
Fixed maturities, trading at fair value		497,108	485,171
Life Funds Withheld Assets | Corporate
Assets
Fixed maturities AFS		1,121,203	1,137,213
Fixed maturities, trading at fair value		1,525,761	1,460,292
Life Funds Withheld Assets | Residential mortgage-backed securities
Assets
Fixed maturities AFS		503	515
Fixed maturities, trading at fair value		893	932
Life Funds Withheld Assets | Commercial mortgage-backed securities
Assets
Fixed maturities AFS		130,070	122,272
Fixed maturities, trading at fair value		7,472	7,580
Life Funds Withheld Assets | Other asset-backed securities
Assets
Fixed maturities AFS		160,750	171,262
Fixed maturities, trading at fair value		43,697	41,337
Recurring
Assets
Total investments available for sale		31,156,798	32,458,436
Cash equivalents	[1]	1,931,733	1,550,707
Other investments	[2]	812,314	792,639
Other assets	[3]	28,664	29,215
Total assets accounted for at fair value		36,738,509	36,920,329
Counterparty netting, assets	[3]	(7,239)	(3,579)
Liabilities
Funds withheld on GreyCastle life retrocession arrangements	[4]	1,085,099	1,112,969
Other liabilities	[3]	40,162	59,317
Total liabilities accounted for at fair value		1,125,261	1,172,286
Counterparty netting, liabilities	[3]	(7,239)	(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]	522,096	279,224
Other investments	[2]	0	0
Other assets	[3]	0	0
Total assets accounted for at fair value		997,728	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		31,122,962	31,876,112
Cash equivalents	[1]	1,409,637	1,271,483
Other investments	[2]	580,946	570,931
Other assets	[3]	17,728	14,467
Total assets accounted for at fair value		35,464,641	35,822,325
Liabilities
Funds withheld on GreyCastle life retrocession arrangements	[4]	1,085,099	1,112,969
Other liabilities	[3]	29,560	44,591
Total liabilities accounted for at fair value		1,114,659	1,157,560
Recurring | Significant other unobservable inputs (Level 3)
Assets
Total investments available for sale		33,836	24,814
Cash equivalents	[1]	0	0
Other investments	[2]	231,368	221,708
Other assets	[3]	18,175	18,327
Total assets accounted for at fair value		283,379	264,849
Liabilities
Funds withheld on GreyCastle life retrocession arrangements	[4]	0	0
Other liabilities	[3]	17,841	18,305
Total liabilities accounted for at fair value		17,841	18,305
Recurring | Excluding Life Funds Withheld Assets
Assets
Fixed maturities AFS		28,648,080	28,901,876
Short-term investments, at fair value	[5]	500,170	815,481
Equity securities, at fair value			713,967
Total investments available for sale		29,148,250	30,431,324
Equity securities, at fair value		637,872
Recurring | Excluding Life Funds Withheld Assets | U.S. Government and Government Agencies
Assets
Fixed maturities AFS		4,187,959	4,362,740
Recurring | Excluding Life Funds Withheld Assets | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities AFS		2,013,162	2,064,640
Recurring | Excluding Life Funds Withheld Assets | Non-U.S. Governments
Assets
Fixed maturities AFS		5,306,651	5,201,581
Recurring | Excluding Life Funds Withheld Assets | Corporate
Assets
Fixed maturities AFS		9,742,670	9,795,133
Recurring | Excluding Life Funds Withheld Assets | Residential mortgage-backed securities
Assets
Fixed maturities AFS		4,529,723	4,724,402
Recurring | Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Assets
Fixed maturities AFS		1,259,519	1,244,017
Recurring | Excluding Life Funds Withheld Assets | Other asset-backed securities
Assets
Fixed maturities AFS		1,608,396	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		474,118
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		28,614,244	28,877,062
Short-term investments, at fair value	[5]	500,170	815,481
Equity securities, at fair value			156,457
Total investments available for sale		29,114,414	29,849,000
Equity securities, at fair value		163,754
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | U.S. Government and Government Agencies
Assets
Fixed maturities AFS		4,182,925	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		2,013,162	2,064,640
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Non-U.S. Governments
Assets
Fixed maturities AFS		5,306,651	5,201,581
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Corporate
Assets
Fixed maturities AFS		9,740,542	9,793,841
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities AFS		4,522,786	4,724,402
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities AFS		1,259,519	1,244,017
Recurring | Excluding Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Other asset-backed securities
Assets
Fixed maturities AFS		1,588,659	1,504,131
Recurring | Excluding Life Funds Withheld Assets | Significant other unobservable inputs (Level 3)
Assets
Fixed maturities AFS		33,836	24,814
Short-term investments, at fair value	[5]	0	0
Equity securities, at fair value			0
Total investments available for sale		33,836	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		5,034	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		2,128	1,292
Recurring | Excluding Life Funds Withheld Assets | Significant other unobservable inputs (Level 3) | Residential mortgage-backed securities
Assets
Fixed maturities AFS		6,937	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		19,737	5,232
Recurring | Life Funds Withheld Assets
Assets
Fixed maturities AFS		2,008,548	2,027,112
Fixed maturities, trading at fair value		2,084,251	2,006,385
Short-term investments, trading at fair value	[5]	7,425	14,965
Total investments trading		2,091,676	2,021,350
Cash equivalents	[1]	79,452	67,982
Recurring | Life Funds Withheld Assets | U.S. Government and Government Agencies
Assets
Fixed maturities AFS		11,616	11,316
Fixed maturities, trading at fair value		9,320	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		584,406	584,534
Fixed maturities, trading at fair value		497,108	485,171
Recurring | Life Funds Withheld Assets | Corporate
Assets
Fixed maturities AFS		1,121,203	1,137,213
Fixed maturities, trading at fair value		1,525,761	1,460,292
Recurring | Life Funds Withheld Assets | Residential mortgage-backed securities
Assets
Fixed maturities AFS		503	515
Fixed maturities, trading at fair value		893	932
Recurring | Life Funds Withheld Assets | Commercial mortgage-backed securities
Assets
Fixed maturities AFS		130,070	122,272
Fixed maturities, trading at fair value		7,472	7,580
Recurring | Life Funds Withheld Assets | Other asset-backed securities
Assets
Fixed maturities AFS		160,750	171,262
Fixed maturities, trading at fair value		43,697	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]	1,514	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		2,008,548	2,027,112
Fixed maturities, trading at fair value		2,084,251	2,006,385
Short-term investments, trading at fair value	[5]	7,425	14,965
Total investments trading		2,091,676	2,021,350
Cash equivalents	[1]	77,938	67,982
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2) | U.S. Government and Government Agencies
Assets
Fixed maturities AFS		11,616	11,316
Fixed maturities, trading at fair value		9,320	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		584,406	584,534
Fixed maturities, trading at fair value		497,108	485,171
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Corporate
Assets
Fixed maturities AFS		1,121,203	1,137,213
Fixed maturities, trading at fair value		1,525,761	1,460,292
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities AFS		503	515
Fixed maturities, trading at fair value		893	932
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities AFS		130,070	122,272
Fixed maturities, trading at fair value		7,472	7,580
Recurring | Life Funds Withheld Assets | Significant other observable inputs (Level 2) | Other asset-backed securities
Assets
Fixed maturities AFS		160,750	171,262
Fixed maturities, trading at fair value		43,697	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 $201.8 million and $198.2 million at March 31, 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 $172.9 million at March 31, 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, Corporate securities, and Other asset-backed securities.