Borrowings - Outstanding Debt Obligations (Details) SFr in Millions, $ in Millions	Sep. 30, 2023 USD ($)	Sep. 30, 2023 EUR (€)	Sep. 30, 2023 GBP (£)	Sep. 30, 2023 CAD ($)	Sep. 30, 2023 CHF (SFr)	Dec. 31, 2022 USD ($)	Dec. 31, 2022 EUR (€)	Dec. 31, 2022 GBP (£)	Dec. 31, 2022 CAD ($)	Dec. 31, 2022 CHF (SFr)	Nov. 17, 2022 USD ($)	Oct. 11, 2022 USD ($)	Sep. 27, 2022 USD ($)	Aug. 12, 2022 USD ($)	Jul. 18, 2022 USD ($)	Jun. 09, 2022 USD ($)	May 03, 2022 USD ($)	Apr. 14, 2022 GBP (£)	Mar. 24, 2022 USD ($)	Jan. 18, 2022 USD ($)	Nov. 22, 2021 USD ($)	Nov. 02, 2021 EUR (€)	Nov. 01, 2021 USD ($)	Sep. 15, 2021 USD ($)	Aug. 17, 2021 USD ($)	Jun. 29, 2021 USD ($)	Jun. 21, 2021 USD ($)
Debt Instrument [Line Items]
Aggregate Principal Committed	$ 34,855,746,000					$ 36,037,871,000
Short-Term Debt	58,400,000					426,800,000
Total	23,366,487,000					26,842,239,000
Total	23,013,800,000					26,493,658,000
Unused Portion	11,489,259,000					9,195,632,000
Amount Available	9,771,671,000					6,141,643,000
Net of unamortized debt issuance costs	85,373,000					102,290,000
Bard Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	1,650,000,000					1,650,000,000
Outstanding Principal	507,074,000	€ 69,000,000	£ 74,600,000	$ 94.1		1,235,414,000	€ 66,000,000	£ 50,000,000	$ 94.1
Net Carrying Value	507,074,000					1,235,414,000
Unused Portion	1,142,926,000					414,586,000
Amount Available	983,358,000					414,586,000
Castle Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	1,600,000,000					1,600,000,000
Outstanding Principal	1,119,717,000	46,400,000	64,400,000	78.9		1,146,600,000	36,000,000	42,400,000	48.8
Net Carrying Value	1,119,717,000					1,146,600,000
Unused Portion	480,283,000					453,400,000
Amount Available	72,323,000					166,697,000
Maroon Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed						300,000,000
Outstanding Principal						300,000,000
Net Carrying Value						300,000,000
Unused Portion						0
Amount Available						0
Summit Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	2,300,000,000					2,300,000,000
Outstanding Principal	531,099,000	26,800,000	101,700,000	154.3		1,691,844,000		6,100,000	132.7
Net Carrying Value	531,099,000					1,691,844,000
Unused Portion	1,768,901,000					608,156,000
Amount Available	1,388,054,000					15,377,000
Denali Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	750,000,000					750,000,000
Outstanding Principal	562,800,000					749,800,000
Net Carrying Value	562,800,000					749,800,000
Unused Portion	187,200,000					200,000
Amount Available	187,200,000					200,000
Bushnell Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	600,000,000					600,000,000
Outstanding Principal	430,000,000					400,000,000
Net Carrying Value	430,000,000					400,000,000
Unused Portion	170,000,000					200,000,000
Amount Available	38,993,000					69,559,000
Granite Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	750,000,000					750,000,000
Outstanding Principal	675,600,000					647,600,000
Net Carrying Value	675,600,000					647,600,000
Unused Portion	74,400,000					102,400,000
Amount Available	22,911,000					102,134,000
Middle Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	1,000,000,000					800,000,000
Outstanding Principal	600,950,000					596,950,000
Net Carrying Value	600,950,000					596,950,000
Unused Portion	399,050,000					203,050,000
Amount Available	270,427,000					203,050,000
Bison Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	1,500,000,000					1,500,000,000
Outstanding Principal	700,000,000					1,182,000,000
Net Carrying Value	700,000,000					1,182,000,000
Unused Portion	800,000,000					318,000,000
Amount Available	800,000,000					233,740,000
Blanca Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	1,500,000,000					1,500,000,000
Outstanding Principal	1,375,100,000					1,081,000,000
Net Carrying Value	1,375,100,000					1,081,000,000
Unused Portion	124,900,000					419,000,000
Amount Available	124,900,000					360,054,000
Windom Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	2,150,000,000					2,000,000,000
Outstanding Principal	1,290,314,000		115,900,000			1,741,465,000		43,600,000
Net Carrying Value	1,290,314,000					1,741,465,000
Unused Portion	859,686,000					258,535,000
Amount Available	859,686,000					258,535,000
Monarch Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	2,000,000,000					2,000,000,000
Outstanding Principal	1,400,400,000					873,400,000
Net Carrying Value	1,400,400,000					873,400,000
Unused Portion	599,600,000					1,126,600,000
Amount Available	349,924,000					968,726,000
Borah Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	400,000,000					400,000,000
Outstanding Principal	80,000,000					223,000,000
Net Carrying Value	80,000,000					223,000,000
Unused Portion	320,000,000					177,000,000
Amount Available	111,944,000					35,951,000
2022-1 BSL WH | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Net Carrying Value						148,000,000
2022-1 BSL WH | Secured debt
Debt Instrument [Line Items]
Aggregate Principal Committed						300,000,000
Outstanding Principal						148,000,000
Unused Portion						152,000,000
Amount Available						152,000,000
Naomi Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	400,000,000					400,000,000									$ 150,000,000
Outstanding Principal	385,000,000					400,000,000
Net Carrying Value	385,000,000					400,000,000
Unused Portion	15,000,000					0
Amount Available	15,000,000					0
Meridian Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	350,000,000					500,000,000
Outstanding Principal	220,000,000					170,000,000
Net Carrying Value	220,000,000					170,000,000
Unused Portion	130,000,000					330,000,000
Amount Available	130,000,000					330,000,000
Haydon Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	250,000,000					500,000,000
Outstanding Principal	49,000,000					49,000,000
Net Carrying Value	49,000,000					49,000,000
Unused Portion	201,000,000					451,000,000
Amount Available	201,000,000					373,519,000
Bear Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	529,375,000	500,000,000				468,494,000
Outstanding Principal	370,660,000	260,800,000	57,000,000			166,031,000	75,000,000	50,700,000
Net Carrying Value	370,660,000					166,031,000
Unused Portion	158,715,000					302,463,000
Amount Available	158,715,000					130,772,000
Revolving Credit Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	5,150,000,000					5,150,000,000
Outstanding Principal	1,092,402,000	€ 6,500,000	£ 419,700,000	$ 168.0	SFr 54.3	1,470,758,000	€ 49,000,000	£ 608,800,000	$ 247.1	SFr 102.6
Net Carrying Value	1,092,402,000					1,470,758,000
Unused Portion	4,057,598,000					3,679,242,000
Amount Available	4,057,236,000					2,326,743,000
June 2024 Notes | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	435,000,000					435,000,000																					$ 435,000,000
Outstanding Principal	435,000,000					435,000,000
Net Carrying Value	422,097,000					413,963,000
Net of unamortized debt issuance costs	940,000					1,912,000
June 2026 Notes | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	400,000,000					400,000,000																			$ 400,000,000
Outstanding Principal	400,000,000					400,000,000
Net Carrying Value	398,104,000					397,611,000
Net of unamortized debt issuance costs	1,896,000					2,389,000
May 2027 Notes | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	625,000,000					625,000,000											$ 625,000,000
Outstanding Principal	625,000,000					625,000,000
Net Carrying Value	587,180,000					596,809,000
Net of unamortized debt issuance costs	1,988,000					2,402,000
October 2027 Notes | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	350,000,000					350,000,000						$ 350,000,000
Outstanding Principal	350,000,000					350,000,000
Net Carrying Value	334,814,000					345,618,000
Net of unamortized debt issuance costs	5,556,000					3,356,000
September 2024 Notes | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	365,000,000					365,000,000																		$ 365,000,000
Outstanding Principal	365,000,000					365,000,000
Net Carrying Value	350,952,000					345,353,000
Net of unamortized debt issuance costs	1,132,000					2,015,000
December 2026 Notes | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	1,250,000,000					1,250,000,000																		$ 900,000,000
Outstanding Principal	1,250,000,000					1,250,000,000
Net Carrying Value	1,152,272,000					1,162,596,000
Net of unamortized debt issuance costs	15,479,000					19,084,000
November 2026 Eurobonds | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	529,375,000					534,975,000																€ 500,000,000
Outstanding Principal	529,375,000					534,975,000
Net Carrying Value	523,982,000					529,223,000
Net of unamortized debt issuance costs	4,943,000					5,752,000
November 2024 Notes | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	500,000,000					500,000,000															$ 500,000,000
Outstanding Principal	500,000,000					500,000,000
Net Carrying Value	477,717,000					471,336,000
Net of unamortized debt issuance costs	1,865,000					3,083,000
March 2027 Notes | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	1,000,000,000					1,000,000,000															$ 1,000,000,000
Outstanding Principal	1,000,000,000					1,000,000,000
Net Carrying Value	990,627,000					988,597,000
Net of unamortized debt issuance costs	9,373,000					11,403,000
January 2025 Notes | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	500,000,000					500,000,000														$ 500,000,000
Outstanding Principal	500,000,000					500,000,000
Net Carrying Value	475,819,000					470,441,000
Net of unamortized debt issuance costs	2,215,000					3,460,000
January 2029 Notes | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	650,000,000					650,000,000														$ 650,000,000
Outstanding Principal	650,000,000					650,000,000
Net Carrying Value	640,581,000					639,254,000
Net of unamortized debt issuance costs	9,419,000					10,746,000
March 2025 Notes | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	900,000,000					900,000,000													$ 900,000,000
Outstanding Principal	900,000,000					900,000,000
Net Carrying Value	858,004,000					853,359,000
Net of unamortized debt issuance costs	4,334,000					6,527,000
April 2026 UK Bonds | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	305,150,000					301,725,000												£ 250,000,000
Outstanding Principal	305,150,000					301,725,000
Net Carrying Value	282,016,000					278,946,000
Net of unamortized debt issuance costs	2,232,000					2,890,000
September 2025 Notes | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	800,000,000					800,000,000					$ 200,000,000		$ 600,000,000
Outstanding Principal	800,000,000					800,000,000
Net Carrying Value	778,124,000					786,332,000
Net of unamortized debt issuance costs	8,550,000					10,952,000
2021-1 BSL Debt
Debt Instrument [Line Items]
Outstanding Principal	819,500,000
2021-1 BSL Debt | Secured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	663,000,000					663,000,000																				$ 876,600,000
Outstanding Principal	663,000,000					663,000,000
Net Carrying Value	662,061,000					661,997,000
Net of unamortized debt issuance costs	939,000					1,003,000
2021-2 Debt
Debt Instrument [Line Items]
Outstanding Principal	603,650,000
2021-2 Debt | Secured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	505,800,000					505,800,000																	$ 603,700,000
Outstanding Principal	505,800,000					505,800,000
Net Carrying Value	504,352,000					504,255,000
Net of unamortized debt issuance costs	1,448,000					1,545,000
MML 2021-1 Debt | Secured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	690,000,000					690,000,000
Outstanding Principal	690,000,000					690,000,000
Net Carrying Value	686,033,000					685,884,000
Net of unamortized debt issuance costs	3,967,000					4,116,000
MML 2022-1 Debt | Secured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	759,000,000					759,000,000
Outstanding Principal	759,000,000					759,000,000
Net Carrying Value	753,913,000					753,600,000
Net of unamortized debt issuance costs	5,087,000					5,400,000
2022-1 BSL Debt
Debt Instrument [Line Items]
Outstanding Principal	589,750,000
2022-1 BSL Debt | Secured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	420,000,000					420,000,000										$ 589,750,000
Outstanding Principal	531,000,000
Outstanding Principal, Net of Eliminations	420,000,000					420,000,000
Net Carrying Value	418,502,000					418,408,000
Net of unamortized debt issuance costs	1,498,000					1,592,000
MML 2022-2 Debt
Debt Instrument [Line Items]
Outstanding Principal	498,105,000													$ 498,100,000
MML 2022-2 Debt | Secured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	300,500,000					300,500,000
Outstanding Principal	341,000,000
Outstanding Principal, Net of Eliminations	300,500,000					300,500,000
Net Carrying Value	297,988,000					297,837,000
Net of unamortized debt issuance costs	2,512,000					2,663,000
Short-Term Borrowings
Debt Instrument [Line Items]
Aggregate Principal Committed	28,546,000					619,377,000
Short-Term Debt	$ 28,546,000					$ 619,377,000