Fair Value Measurements - Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Debt Instrument [Line Items]
Short-Term Borrowings	$ 28,546	$ 619,377
Total	22,755,058	26,030,878
Bard Peak Funding Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fair Value	507,074	1,235,414
Castle Peak Funding Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fair Value	1,119,717	1,146,600
Maroon Peak Funding Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fair Value	0	300,000
Summit Peak Funding Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fair Value	531,099	1,691,844
Denali Peak Funding Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fair Value	562,800	749,800
Bushnell Peak Funding Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fair Value	430,000	400,000
Granite Peak Funding Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fair Value	675,600	647,600
Middle Peak Funding Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fair Value	600,950	596,950
Bison Peak Funding Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fair Value	700,000	1,182,000
Blanca Peak Funding Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fair Value	1,375,100	1,081,000
Windom Peak Funding Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fair Value	1,290,314	1,741,465
Monarch Peak Funding Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fair Value	1,400,400	873,400
Borah Peak Funding Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fair Value	80,000	223,000
2022-1 BSL WH | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fair Value	0	148,000
Naomi Peak Funding Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fair Value	385,000	400,000
Meridian Peak Funding Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fair Value	220,000	170,000
Haydon Peak Funding Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fair Value	49,000	49,000
Bear Peak Funding Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fair Value	370,660	166,031
Revolving Credit Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fair Value	1,092,402	1,470,758
June 2024 Notes | Unsecured debt
Debt Instrument [Line Items]
Fair Value	421,534	406,886
June 2026 Notes | Unsecured debt
Debt Instrument [Line Items]
Fair Value	364,187	352,646
May 2027 Notes | Unsecured debt
Debt Instrument [Line Items]
Fair Value	590,515	586,924
October 2027 Notes | Unsecured debt
Debt Instrument [Line Items]
Fair Value	351,905	353,434
September 2024 Notes | Unsecured debt
Debt Instrument [Line Items]
Fair Value	348,005	336,654
December 2026 Notes | Unsecured debt
Debt Instrument [Line Items]
Fair Value	1,084,845	1,053,132
November 2026 Eurobonds | Unsecured debt
Debt Instrument [Line Items]
Fair Value	481,579	449,149
November 2024 Notes | Unsecured debt
Debt Instrument [Line Items]
Fair Value	475,577	459,089
March 2027 Notes | Unsecured debt
Debt Instrument [Line Items]
Fair Value	875,570	856,640
January 2025 Notes | Unsecured debt
Debt Instrument [Line Items]
Fair Value	474,342	462,125
January 2029 Notes | Unsecured debt
Debt Instrument [Line Items]
Fair Value	566,167	550,113
March 2025 Notes | Unsecured debt
Debt Instrument [Line Items]
Fair Value	873,731	866,280
April 2026 UK Bonds | Unsecured debt
Debt Instrument [Line Items]
Fair Value	292,022	272,657
September 2025 Notes | Unsecured debt
Debt Instrument [Line Items]
Fair Value	801,647	795,092
2021-1 BSL Debt | Secured debt
Debt Instrument [Line Items]
Fair Value	663,000	663,000
2021-2 Debt | Secured debt
Debt Instrument [Line Items]
Fair Value	505,800	505,800
MML 2021-1 Debt | Secured debt
Debt Instrument [Line Items]
Fair Value	690,000	690,000
MML 2022-1 Debt | Secured debt
Debt Instrument [Line Items]
Fair Value	756,272	759,000
2022-1 BSL Debt | Secured debt
Debt Instrument [Line Items]
Fair Value	420,000	420,000
MML 2022-2 Debt | Secured debt
Debt Instrument [Line Items]
Fair Value	$ 299,698	$ 300,018