Fair Value Measurements - Schedule of assets and liabilities that were accounted for at fair value (Details) - USD ($) $ in Thousands		Mar. 31, 2017	Dec. 31, 2016
Assets
Fixed maturities, at fair value		$ 30,651,790	$ 30,256,602
Equity securities, at fair value		1,094,509	1,037,331
Short-term investments, at fair value		260,153	625,193
Total investments available for sale		32,006,452	31,919,126
Fixed maturities, trading at fair value		1,700,108	1,617,014
Short-term investments, trading at fair value		754	9,563
Total investments trading		1,700,862	1,626,577
Collateral and counterparty netting, assets	[1],[2]	(8,565)	(17,947)
Liabilities
Funds withheld on life retrocession arrangements (net of future policy benefit reserves recoverable)		954,327	998,968
Collateral and counterparty nettings, liabilities	[1],[2]	(8,565)	(17,947)
Recurring
Assets
Total investments available for sale		32,006,452	31,919,126
Cash equivalents	[3]	1,785,866	1,669,652
Other investments	[4]	724,930	699,402
Other assets	[2]	22,420	89,549
Total assets accounted for at fair value		36,326,993	36,077,104
Liabilities
Funds withheld on life retrocession arrangements (net of future policy benefit reserves recoverable)	[5]	977,942	937,721
Other liabilities	[2]	39,222	46,757
Total liabilities accounted for at fair value		1,017,164	984,478
Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Total investments available for sale		942,865	898,664
Cash equivalents	[3]	301,491	573,398
Other investments	[4]	0
Other assets	[2]	0	0
Total assets accounted for at fair value		1,244,356	1,472,062
Liabilities
Funds withheld on life retrocession arrangements (net of future policy benefit reserves recoverable)	[5]	0	0
Other liabilities	[2]	0	0
Total liabilities accounted for at fair value		0	0
Recurring | Significant other observable inputs (Level 2)
Assets
Total investments available for sale		31,015,283	30,974,396
Cash equivalents	[3]	1,484,375	1,096,254
Other investments	[4]	520,965	493,874
Other assets	[2]	12,425	87,655
Total assets accounted for at fair value		34,820,373	34,351,554
Liabilities
Funds withheld on life retrocession arrangements (net of future policy benefit reserves recoverable)	[5]	977,942	937,721
Other liabilities	[2]	29,390	44,865
Total liabilities accounted for at fair value		1,007,332	982,586
Recurring | Significant other unobservable inputs (Level 3)
Assets
Total investments available for sale		48,304	46,066
Cash equivalents	[3]	0	0
Other investments	[4]	203,965	205,528
Other assets	[2]	18,560	19,841
Total assets accounted for at fair value		270,829	271,435
Liabilities
Funds withheld on life retrocession arrangements (net of future policy benefit reserves recoverable)	[5]	0	0
Other liabilities	[2]	18,397	19,839
Total liabilities accounted for at fair value		18,397	19,839
Excluding Life Funds Withheld Assets
Assets
Fixed maturities, at fair value		28,440,809	27,948,560
Equity securities, at fair value		1,094,509	1,037,331
Short-term investments, at fair value		260,153	625,193
Total investments available for sale		29,795,471	29,611,084
Excluding Life Funds Withheld Assets | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		4,203,353	3,894,388
Excluding Life Funds Withheld Assets | Corporate
Assets
Fixed maturities, at fair value		10,579,366	10,134,729
Excluding Life Funds Withheld Assets | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		4,377,435	4,492,625
Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		700,197	665,186
Excluding Life Funds Withheld Assets | Other asset-backed securities
Assets
Fixed maturities, at fair value		1,249,691	1,253,388
Excluding Life Funds Withheld Assets | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, at fair value		2,350,859	2,478,112
Excluding Life Funds Withheld Assets | Non-U.S. Governments
Assets
Fixed maturities, at fair value		4,979,908	5,030,132
Excluding Life Funds Withheld Assets | Recurring
Assets
Fixed maturities, at fair value		28,440,809	27,948,560
Equity securities, at fair value		1,094,509	1,037,331
Short-term investments, at fair value	[6]	260,153	625,193
Total investments available for sale		29,795,471	29,611,084
Excluding Life Funds Withheld Assets | Recurring | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		4,203,353	3,894,388
Excluding Life Funds Withheld Assets | Recurring | Corporate
Assets
Fixed maturities, at fair value		10,579,366	10,134,729
Excluding Life Funds Withheld Assets | Recurring | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		4,377,435	4,492,625
Excluding Life Funds Withheld Assets | Recurring | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		700,197	665,186
Excluding Life Funds Withheld Assets | Recurring | Other asset-backed securities
Assets
Fixed maturities, at fair value		1,249,691	1,253,388
Excluding Life Funds Withheld Assets | Recurring | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, at fair value		2,350,859	2,478,112
Excluding Life Funds Withheld Assets | Recurring | Non-U.S. Governments
Assets
Fixed maturities, at fair value		4,979,908	5,030,132
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		942,865	898,664
Short-term investments, at fair value	[6]	0	0
Total investments available for sale		942,865	898,664
Excluding Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | U.S. Government and Government Agencies
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	0
Excluding Life Funds Withheld Assets | Recurring | Quoted prices in active markets for identical assets (Level 1) | Residential 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) | 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) | 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, municipalities 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. Governments
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		28,404,170	27,902,494
Equity securities, at fair value		151,644	138,667
Short-term investments, at fair value	[6]	248,488	625,193
Total investments available for sale		28,804,302	28,666,354
Excluding Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		4,184,045	3,869,637
Excluding Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Corporate
Assets
Fixed maturities, at fair value		10,569,467	10,114,644
Excluding Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		4,374,054	4,492,625
Excluding Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		700,096	665,087
Excluding Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Other asset-backed securities
Assets
Fixed maturities, at fair value		1,245,741	1,252,257
Excluding Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, at fair value		2,350,859	2,478,112
Excluding Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Non-U.S. Governments
Assets
Fixed maturities, at fair value		4,979,908	5,030,132
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3)
Assets
Fixed maturities, at fair value		36,639	46,066
Equity securities, at fair value		0	0
Short-term investments, at fair value	[6]	11,665	0
Total investments available for sale		48,304	46,066
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		19,308	24,751
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Corporate
Assets
Fixed maturities, at fair value		9,899	20,085
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		3,381	0
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		101	99
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Other asset-backed securities
Assets
Fixed maturities, at fair value		3,950	1,131
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | US States, municipalities 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. Governments
Assets
Fixed maturities, at fair value		0	0
Life Funds Withheld Assets
Assets
Fixed maturities, at fair value		2,210,981	2,308,042
Fixed maturities, trading at fair value		1,700,108	1,617,014
Short-term investments, trading at fair value		754	9,563
Total investments trading		1,700,862	1,626,577
Life Funds Withheld Assets | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		10,732	10,577
Fixed maturities, trading at fair value		16,340	14,708
Life Funds Withheld Assets | Corporate
Assets
Fixed maturities, at fair value		1,395,551	1,454,608
Fixed maturities, trading at fair value		1,167,081	1,111,205
Life Funds Withheld Assets | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		22,338	22,380
Fixed maturities, trading at fair value		930	961
Life Funds Withheld Assets | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		93,810	94,240
Fixed maturities, trading at fair value		5,550	5,526
Life Funds Withheld Assets | Other asset-backed securities
Assets
Fixed maturities, at fair value		99,928	101,560
Fixed maturities, trading at fair value		40,484	39,446
Life Funds Withheld Assets | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, trading at fair value		126	224
Life Funds Withheld Assets | Non-U.S. Governments
Assets
Fixed maturities, at fair value		588,622	624,677
Fixed maturities, trading at fair value		469,597	444,944
Life Funds Withheld Assets | Recurring
Assets
Fixed maturities, at fair value		2,210,981	2,308,042
Fixed maturities, trading at fair value		1,700,108	1,617,014
Short-term investments, trading at fair value	[6]	754	9,563
Total investments trading		1,700,862	1,626,577
Cash equivalents	[3]	86,463	72,798
Life Funds Withheld Assets | Recurring | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		10,732	10,577
Fixed maturities, trading at fair value		16,340	14,708
Life Funds Withheld Assets | Recurring | Corporate
Assets
Fixed maturities, at fair value		1,395,551	1,454,608
Fixed maturities, trading at fair value		1,167,081	1,111,205
Life Funds Withheld Assets | Recurring | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		22,338	22,380
Fixed maturities, trading at fair value		930	961
Life Funds Withheld Assets | Recurring | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		93,810	94,240
Fixed maturities, trading at fair value		5,550	5,526
Life Funds Withheld Assets | Recurring | Other asset-backed securities
Assets
Fixed maturities, at fair value		99,928	101,560
Fixed maturities, trading at fair value		40,484	39,446
Life Funds Withheld Assets | Recurring | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, at fair value			0
Fixed maturities, trading at fair value		126	224
Life Funds Withheld Assets | Recurring | Non-U.S. Governments
Assets
Fixed maturities, at fair value		588,622	624,677
Fixed maturities, trading at fair value		469,597	444,944
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
Short-term investments, trading at fair value	[6]	0	0
Total investments trading		0	0
Cash equivalents	[3]	0	0
Life Funds Withheld Assets | Recurring | 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
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) | Residential mortgage-backed securities
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) | Commercial mortgage-backed securities
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) | Other asset-backed securities
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) | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, at fair value			0
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, 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, at fair value		2,210,981	2,308,042
Fixed maturities, trading at fair value		1,700,108	1,617,014
Short-term investments, trading at fair value	[6]	754	9,563
Total investments trading		1,700,862	1,626,577
Cash equivalents	[3]	86,463	72,798
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		10,732	10,577
Fixed maturities, trading at fair value		16,340	14,708
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Corporate
Assets
Fixed maturities, at fair value		1,395,551	1,454,608
Fixed maturities, trading at fair value		1,167,081	1,111,205
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		22,338	22,380
Fixed maturities, trading at fair value		930	961
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		93,810	94,240
Fixed maturities, trading at fair value		5,550	5,526
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Other asset-backed securities
Assets
Fixed maturities, at fair value		99,928	101,560
Fixed maturities, trading at fair value		40,484	39,446
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, at fair value			0
Fixed maturities, trading at fair value		126	224
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Non-U.S. Governments
Assets
Fixed maturities, at fair value		588,622	624,677
Fixed maturities, trading at fair value		469,597	444,944
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
Short-term investments, trading at fair value	[6]	0	0
Total investments trading		0	0
Cash equivalents	[3]	0	0
Life Funds Withheld Assets | Recurring | 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
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) | Residential mortgage-backed securities
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) | Commercial mortgage-backed securities
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) | Other asset-backed securities
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) | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, at fair value			0
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, at fair value		0	0
Fixed maturities, trading at fair value		$ 0	$ 0

[1] Derivative instruments in an asset or liability position are included within Other assets or Other liabilities, respectively, in the balance sheets on a net basis where the Company has both a legal right of offset and the intention to settle the contracts on a net basis. The Company often enters into different types of derivative contracts with a single counterparty and these contracts are covered under netting agreements.

[2] 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 5, "Derivative Instruments.

[3] 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.

[4] Excluded from Other Investments are certain investments which are measured using net asset value as a practical expedient in the amount of $283.0 million and $322.9 million at March 31, 2017 and December 31, 2016, respectively. Based on new accounting guidance, these investments are excluded from the fair value hierarchy table. In addition, the Other investments balance excludes a certain payment obligation. This investment, which totaled $142.5 million at March 31, 2017 and $142.3 million at December 31, 2016, is 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.

[5] 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.

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