Fair Value Measurements - Schedule of assets and liabilities that were accounted for at fair value (Details) - USD ($) $ in Thousands		Jun. 30, 2017	Dec. 31, 2016
Assets
Fixed maturities, at fair value		$ 30,810,724	$ 30,256,602
Equity securities, at fair value		1,174,500	1,037,331
Short-term investments, at fair value		617,894	625,193
Total investments available for sale		32,603,118	31,919,126
Fixed maturities, trading at fair value		1,772,877	1,617,014
Short-term investments, trading at fair value		0	9,563
Total investments trading		1,772,877	1,626,577
Collateral and counterparty netting, assets	[1]	(2,460)	(17,947)
Liabilities
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable: 2017 - $3,170,975; 2016 - $3,072,615)		918,304	998,968
Collateral and counterparty nettings, liabilities	[1]	(2,460)	(17,947)
Recurring
Assets
Total investments available for sale		32,603,118	31,919,126
Cash equivalents	[2]	1,380,464	1,669,652
Other investments	[3]	730,299	699,402
Other assets	[1]	28,527	89,549
Total assets accounted for at fair value		36,593,485	36,077,104
Liabilities
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable: 2017 - $3,170,975; 2016 - $3,072,615)	[4]	1,010,974	937,721
Other liabilities	[1]	88,074	46,757
Total liabilities accounted for at fair value		1,099,048	984,478
Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Total investments available for sale		983,785	898,664
Cash equivalents	[2]	627,621	573,398
Other investments	[3]	0	0
Other assets	[1]	0	0
Total assets accounted for at fair value		1,611,406	1,472,062
Liabilities
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable: 2017 - $3,170,975; 2016 - $3,072,615)	[4]	0	0
Other liabilities	[1]	0	0
Total liabilities accounted for at fair value		0	0
Recurring | Significant other observable inputs (Level 2)
Assets
Total investments available for sale		31,592,118	30,974,396
Cash equivalents	[2]	752,843	1,096,254
Other investments	[3]	509,951	493,874
Other assets	[1]	12,446	87,655
Total assets accounted for at fair value		34,718,435	34,351,554
Liabilities
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable: 2017 - $3,170,975; 2016 - $3,072,615)	[4]	1,010,974	937,721
Other liabilities	[1]	72,533	44,865
Total liabilities accounted for at fair value		1,083,507	982,586
Recurring | Significant other unobservable inputs (Level 3)
Assets
Total investments available for sale		27,215	46,066
Cash equivalents	[2]	0	0
Other investments	[3]	220,348	205,528
Other assets	[1]	18,541	19,841
Total assets accounted for at fair value		266,104	271,435
Liabilities
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable: 2017 - $3,170,975; 2016 - $3,072,615)	[4]	0	0
Other liabilities	[1]	18,001	19,839
Total liabilities accounted for at fair value		18,001	19,839
Excluding Life Funds Withheld Assets
Assets
Fixed maturities, at fair value		28,641,939	27,948,560
Equity securities, at fair value		1,174,500	1,037,331
Short-term investments, at fair value		617,894	625,193
Total investments available for sale		30,434,333	29,611,084
Excluding Life Funds Withheld Assets | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		4,355,401	3,894,388
Excluding Life Funds Withheld Assets | Corporate
Assets
Fixed maturities, at fair value		10,542,915	10,134,729
Excluding Life Funds Withheld Assets | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		3,954,318	4,492,625
Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		795,453	665,186
Excluding Life Funds Withheld Assets | Other asset-backed securities
Assets
Fixed maturities, at fair value		1,587,056	1,253,388
Excluding Life Funds Withheld Assets | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, at fair value		2,283,272	2,478,112
Excluding Life Funds Withheld Assets | Non-U.S. Governments
Assets
Fixed maturities, at fair value		5,123,524	5,030,132
Excluding Life Funds Withheld Assets | Recurring
Assets
Fixed maturities, at fair value		28,641,939	27,948,560
Equity securities, at fair value		1,174,500	1,037,331
Short-term investments, at fair value	[5]	617,894	625,193
Total investments available for sale		30,434,333	29,611,084
Excluding Life Funds Withheld Assets | Recurring | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		4,355,401	3,894,388
Excluding Life Funds Withheld Assets | Recurring | Corporate
Assets
Fixed maturities, at fair value		10,542,915	10,134,729
Excluding Life Funds Withheld Assets | Recurring | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		3,954,318	4,492,625
Excluding Life Funds Withheld Assets | Recurring | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		795,453	665,186
Excluding Life Funds Withheld Assets | Recurring | Other asset-backed securities
Assets
Fixed maturities, at fair value		1,587,056	1,253,388
Excluding Life Funds Withheld Assets | Recurring | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, at fair value		2,283,272	2,478,112
Excluding Life Funds Withheld Assets | Recurring | Non-U.S. Governments
Assets
Fixed maturities, at fair value		5,123,524	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		983,785	898,664
Short-term investments, at fair value	[5]	0	0
Total investments available for sale		983,785	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		28,614,727	27,902,494
Equity securities, at fair value		190,715	138,667
Short-term investments, at fair value	[5]	617,891	625,193
Total investments available for sale		29,423,333	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		4,336,342	3,869,637
Excluding Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Corporate
Assets
Fixed maturities, at fair value		10,541,731	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		3,954,318	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		795,375	665,087
Excluding Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Other asset-backed securities
Assets
Fixed maturities, at fair value		1,580,165	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		2,283,272	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		5,123,524	5,030,132
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3)
Assets
Fixed maturities, at fair value		27,212	46,066
Equity securities, at fair value		0	0
Short-term investments, at fair value	[5]	3	0
Total investments available for sale		27,215	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		19,059	24,751
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Corporate
Assets
Fixed maturities, at fair value		1,184	20,085
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		0	0
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		78	99
Excluding Life Funds Withheld Assets | Recurring | Significant other unobservable inputs (Level 3) | Other asset-backed securities
Assets
Fixed maturities, at fair value		6,891	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		2,168,785	2,308,042
Fixed maturities, trading at fair value		1,772,877	1,617,014
Short-term investments, trading at fair value		0	9,563
Total investments trading		1,772,877	1,626,577
Life Funds Withheld Assets | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		10,923	10,577
Fixed maturities, trading at fair value		16,308	14,708
Life Funds Withheld Assets | Corporate
Assets
Fixed maturities, at fair value		1,247,327	1,454,608
Fixed maturities, trading at fair value		1,243,221	1,111,205
Life Funds Withheld Assets | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		627	22,380
Fixed maturities, trading at fair value		838	961
Life Funds Withheld Assets | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		121,094	94,240
Fixed maturities, trading at fair value		7,318	5,526
Life Funds Withheld Assets | Other asset-backed securities
Assets
Fixed maturities, at fair value		193,078	101,560
Fixed maturities, trading at fair value		41,633	39,446
Life Funds Withheld Assets | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, trading at fair value		70	224
Life Funds Withheld Assets | Non-U.S. Governments
Assets
Fixed maturities, at fair value		595,736	624,677
Fixed maturities, trading at fair value		463,489	444,944
Life Funds Withheld Assets | Recurring
Assets
Fixed maturities, at fair value		2,168,785	2,308,042
Fixed maturities, trading at fair value		1,772,877	1,617,014
Short-term investments, trading at fair value	[5]	0	9,563
Total investments trading		1,772,877	1,626,577
Cash equivalents	[2]	78,200	72,798
Life Funds Withheld Assets | Recurring | U.S. Government and Government Agencies
Assets
Fixed maturities, at fair value		10,923	10,577
Fixed maturities, trading at fair value		16,308	14,708
Life Funds Withheld Assets | Recurring | Corporate
Assets
Fixed maturities, at fair value		1,247,327	1,454,608
Fixed maturities, trading at fair value		1,243,221	1,111,205
Life Funds Withheld Assets | Recurring | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		627	22,380
Fixed maturities, trading at fair value		838	961
Life Funds Withheld Assets | Recurring | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		121,094	94,240
Fixed maturities, trading at fair value		7,318	5,526
Life Funds Withheld Assets | Recurring | Other asset-backed securities
Assets
Fixed maturities, at fair value		193,078	101,560
Fixed maturities, trading at fair value		41,633	39,446
Life Funds Withheld Assets | Recurring | US States, municipalities and political subdivisions of the States
Assets
Fixed maturities, at fair value		0	0
Fixed maturities, trading at fair value		70	224
Life Funds Withheld Assets | Recurring | Non-U.S. Governments
Assets
Fixed maturities, at fair value		595,736	624,677
Fixed maturities, trading at fair value		463,489	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	[5]	0	0
Total investments trading		0	0
Cash equivalents	[2]	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	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		2,168,785	2,308,042
Fixed maturities, trading at fair value		1,772,877	1,617,014
Short-term investments, trading at fair value	[5]	0	9,563
Total investments trading		1,772,877	1,626,577
Cash equivalents	[2]	78,200	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		10,923	10,577
Fixed maturities, trading at fair value		16,308	14,708
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Corporate
Assets
Fixed maturities, at fair value		1,247,327	1,454,608
Fixed maturities, trading at fair value		1,243,221	1,111,205
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities, at fair value		627	22,380
Fixed maturities, trading at fair value		838	961
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities, at fair value		121,094	94,240
Fixed maturities, trading at fair value		7,318	5,526
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Other asset-backed securities
Assets
Fixed maturities, at fair value		193,078	101,560
Fixed maturities, trading at fair value		41,633	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	0
Fixed maturities, trading at fair value		70	224
Life Funds Withheld Assets | Recurring | Significant other observable inputs (Level 2) | Non-U.S. Governments
Assets
Fixed maturities, at fair value		595,736	624,677
Fixed maturities, trading at fair value		463,489	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	[5]	0	0
Total investments trading		0	0
Cash equivalents	[2]	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	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] 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.

[2] 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.

[3] Excluded from Other Investments are certain investments which are measured using net asset value as a practical expedient in the amount of $247.8 million and $322.9 million at June 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 $148.6 million at June 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.

[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.