Borrowings - Outstanding Debt Obligations (Details) SFr in Millions, $ in Millions	Sep. 30, 2022 USD ($)	Sep. 30, 2022 EUR (€)	Sep. 30, 2022 GBP (£)	Sep. 30, 2022 CAD ($)	Sep. 30, 2022 CHF (SFr)	Sep. 22, 2022 USD ($)	Aug. 19, 2022 USD ($)	Aug. 16, 2022 USD ($)	Jul. 18, 2022 USD ($)	Jun. 09, 2022 USD ($)	Jun. 07, 2022 USD ($)	May 03, 2022 USD ($)	May 01, 2022 USD ($)	Apr. 14, 2022 GBP (£)	Apr. 04, 2022 USD ($)	Mar. 24, 2022 USD ($)	Mar. 18, 2022 USD ($)	Mar. 17, 2022 USD ($)	Mar. 09, 2022 USD ($)	Feb. 25, 2022 USD ($)	Jan. 18, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2021 GBP (£)	Dec. 31, 2021 CAD ($)	Dec. 21, 2021 USD ($)	Dec. 15, 2021 USD ($)	Dec. 06, 2021 USD ($)	Nov. 22, 2021 USD ($)	Nov. 18, 2021 USD ($)	Nov. 03, 2021 USD ($)	Nov. 02, 2021 EUR (€)	Nov. 01, 2021 USD ($)	Oct. 29, 2021 USD ($)	Oct. 22, 2021 USD ($)	Oct. 20, 2021 USD ($)	Sep. 30, 2021 USD ($)	Sep. 15, 2021 USD ($)	Sep. 02, 2021 USD ($)	Aug. 17, 2021 USD ($)	Aug. 16, 2021 USD ($)	Jul. 23, 2021 USD ($)	Jul. 15, 2021 USD ($)	Jun. 30, 2021 USD ($)	Jun. 29, 2021 USD ($)	Jun. 21, 2021 USD ($)	Jun. 17, 2021 USD ($)	May 18, 2021 USD ($)	May 12, 2021 USD ($)	Mar. 23, 2021 USD ($)	Mar. 15, 2021 USD ($)	Mar. 03, 2021 USD ($)	Feb. 26, 2021 USD ($)	Jan. 28, 2021 USD ($)	Jan. 08, 2021 USD ($)	Oct. 11, 2018 USD ($)
Debt Instrument [Line Items]
Aggregate Principal Committed	$ 35,414,783,000																					$ 23,121,914,000
Short-Term Borrowings	844,083,000																					718,156,000
Total	27,543,087,000																					18,301,460,000
Total	27,043,081,000																					18,239,934,000
Unused Portion	7,871,697,000																					4,820,454,000
Amount Available	6,089,299,000																					2,897,763,000
Unamortized debt issuance costs	100,245,000																					61,526,000
Securities Sold under Agreements to Repurchase
Debt Instrument [Line Items]
Short-Term Borrowings	844,100,000
Bard Peak Funding Facility
Debt Instrument [Line Items]
Net Carrying Value	1,449,196,000																					879,000,000
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				$ 1,700,000,000			$ 2,000,000,000				$ 1,500,000,000								$ 1,000,000,000									$ 600,000,000
Outstanding Principal	1,449,196,000	€ 36,000,000		$ 39.2																		879,000,000
Net Carrying Value	1,449,196,000																					879,000,000
Unused Portion	200,804,000																					771,000,000
Amount Available	200,804,000																					0
Castle Peak Funding Facility
Debt Instrument [Line Items]
Net Carrying Value	1,195,027,000																					1,171,809,000
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			$ 1,600,000,000																	$ 1,300,000,000								$ 800,000,000				$ 200,000,000
Outstanding Principal	1,195,027,000	36,000,000	£ 42,400,000	60.0																		1,171,809,000	£ 42,400,000	$ 60.0
Net Carrying Value	1,195,027,000																					1,171,809,000
Unused Portion	404,973,000																					428,191,000
Amount Available	327,034,000																					131,041,000
Maroon Peak Funding Facility
Debt Instrument [Line Items]
Net Carrying Value	800,000,000																					483,952,000
Maroon Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	800,000,000																			$ 800,000,000		700,000,000																						$ 700,000,000					$ 1,000,000,000			$ 560,000,000	$ 500,000,000
Outstanding Principal	800,000,000																					483,952,000
Net Carrying Value	800,000,000																					483,952,000
Unused Portion	0																					216,048,000
Amount Available	0																					216,048,000
Summit Peak Funding Facility
Debt Instrument [Line Items]
Net Carrying Value	1,535,572,000																					1,643,154,000
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				2,000,000,000											$ 2,000,000,000															$ 1,000,000,000			$ 500,000,000
Outstanding Principal	1,535,572,000		6,100,000	99.2																		1,643,154,000		60.0
Net Carrying Value	1,535,572,000																					1,643,154,000
Unused Portion	764,428,000																					356,846,000
Amount Available	111,342,000																					86,767,000
Denali Peak Funding Facility
Debt Instrument [Line Items]
Net Carrying Value	749,800,000																					668,400,000
Denali Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	750,000,000																		$ 750,000,000			675,000,000													$ 675,000,000	$ 600,000,000																			$ 200,000,000
Outstanding Principal	749,800,000																					668,400,000
Net Carrying Value	749,800,000																					668,400,000
Unused Portion	200,000																					6,600,000
Amount Available	200,000																					6,600,000
Bushnell Peak Funding Facility
Debt Instrument [Line Items]
Net Carrying Value	600,000,000																					395,500,000
Bushnell Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	600,000,000																					600,000,000					$ 600,000,000																					$ 425,000,000
Outstanding Principal	600,000,000																					395,500,000
Net Carrying Value	600,000,000																					395,500,000
Unused Portion	0																					204,500,000
Amount Available	0																					98,376,000
Granite Peak Funding Facility
Debt Instrument [Line Items]
Net Carrying Value	562,700,000																					248,000,000
Granite Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	750,000,000										$ 750,000,000											250,000,000																								$ 250,000,000
Outstanding Principal	562,700,000																					248,000,000
Net Carrying Value	562,700,000																					248,000,000
Unused Portion	187,300,000																					2,000,000
Amount Available	113,734,000																					2,000,000
Middle Peak Funding Facility
Debt Instrument [Line Items]
Net Carrying Value	599,950,000																					799,550,000
Middle Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	800,000,000																					800,000,000												$ 800,000,000									$ 500,000,000
Outstanding Principal	599,950,000																					799,550,000
Net Carrying Value	599,950,000																					799,550,000
Unused Portion	200,050,000																					450,000
Amount Available	200,050,000																					68,000
Bison Peak Funding Facility
Debt Instrument [Line Items]
Net Carrying Value	1,467,300,000																					1,320,800,000
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														$ 1,500,000,000					$ 1,000,000,000
Outstanding Principal	1,467,300,000																					1,320,800,000
Net Carrying Value	1,467,300,000																					1,320,800,000
Unused Portion	32,700,000																					179,200,000
Amount Available	32,700,000																					69,364,000
Blanca Peak Funding Facility
Debt Instrument [Line Items]
Net Carrying Value	1,000,000,000																					892,800,000
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															1,000,000,000																		$ 1,000,000,000
Outstanding Principal	1,000,000,000																					892,800,000
Net Carrying Value	1,000,000,000																					892,800,000
Unused Portion	500,000,000																					107,200,000
Amount Available	380,923,000																					107,200,000
Windom Peak Funding Facility
Debt Instrument [Line Items]
Net Carrying Value	1,612,648,000																					989,759,000
Windom Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	2,000,000,000																$ 2,000,000,000					1,000,000,000																$ 1,000,000,000
Outstanding Principal	1,612,648,000		43,600,000																			989,759,000	43,600,000
Net Carrying Value	1,612,648,000																					989,759,000
Unused Portion	387,352,000																					10,241,000
Amount Available	386,613,000																					6,471,000
Monarch Peak Funding Facility
Debt Instrument [Line Items]
Net Carrying Value	1,123,400,000																					567,400,000
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									1,000,000,000								$ 1,000,000,000
Outstanding Principal	1,123,400,000																					567,400,000
Net Carrying Value	1,123,400,000																					567,400,000
Unused Portion	876,600,000																					432,600,000
Amount Available	46,780,000																					68,250,000
Borah Peak Funding Facility
Debt Instrument [Line Items]
Net Carrying Value	381,000,000																					0
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	381,000,000
Net Carrying Value	381,000,000
Unused Portion	19,000,000
Amount Available	19,000,000
Naomi Peak Funding Facility
Debt Instrument [Line Items]
Net Carrying Value	340,000,000																					0
Naomi Peak Funding Facility | Line of Credit
Debt Instrument [Line Items]
Aggregate Principal Committed									$ 400,000,000
Naomi Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	400,000,000								$ 150,000,000
Outstanding Principal	340,000,000
Net Carrying Value	340,000,000
Unused Portion	60,000,000
Amount Available	60,000,000
Meridian Peak Funding Facility
Debt Instrument [Line Items]
Net Carrying Value	100,000,000																					0
Meridian Peak Funding Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	500,000,000							$ 500,000,000
Outstanding Principal	100,000,000
Net Carrying Value	100,000,000
Unused Portion	400,000,000
Amount Available	400,000,000
2022-1 BSL WH
Debt Instrument [Line Items]
Net Carrying Value	168,000,000																					0
2022-1 BSL WH | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	400,000,000
Outstanding Principal	168,000,000
Net Carrying Value	168,000,000
Unused Portion	232,000,000
Amount Available	232,000,000
Revolving Credit Facility | Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	5,150,000,000																					3,250,000,000																									$ 3,000,000,000
Outstanding Principal	1,543,710,000	€ 75,500,000	£ 660,300,000	$ 323.6	SFr 105.1																	1,144,422,000	£ 156,900,000	$ 46.8
Net Carrying Value	1,543,710,000																					1,144,422,000
Unused Portion	3,606,290,000																					2,105,578,000
Amount Available	3,578,119,000																					2,105,578,000
June 2024 Notes
Debt Instrument [Line Items]
Net Carrying Value	413,517,000																					431,854,000
Unamortized debt issuance costs	2,200,000																					3,100,000
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	413,517,000																					431,854,000
June 2026 Notes
Debt Instrument [Line Items]
Net Carrying Value	397,444,000																					396,952,000
Unamortized debt issuance costs	2,500,000																					3,000,000
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	397,444,000																					396,952,000
September 2024 Notes
Debt Instrument [Line Items]
Net Carrying Value	344,879,000																					361,805,000
Unamortized debt issuance costs	2,300,000																					3,200,000
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	344,879,000																					361,805,000
December 2026 Notes
Debt Instrument [Line Items]
Net Carrying Value	1,155,655,000																					1,227,844,000
Unamortized debt issuance costs	19,600,000																					22,200,000
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	1,155,655,000																					1,227,844,000
November 2026 Eurobonds
Debt Instrument [Line Items]
Net Carrying Value	484,402,000																					563,695,000
Unamortized debt issuance costs	6,100,000																					6,300,000
November 2026 Eurobonds | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	580,475,000																					569,958,000									€ 500,000,000
Outstanding Principal	580,475,000																					569,958,000
Net Carrying Value	484,402,000																					563,695,000
November 2024 Notes
Debt Instrument [Line Items]
Net Carrying Value	470,872,000																					496,054,000
Unamortized debt issuance costs	3,100,000																					3,900,000
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	470,872,000																					496,054,000
March 2027 Notes
Debt Instrument [Line Items]
Net Carrying Value	988,722,000																					987,298,000
Unamortized debt issuance costs	11,300,000																					12,700,000
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	988,722,000																					987,298,000
January 2025 Notes
Debt Instrument [Line Items]
Net Carrying Value	469,326,000																					0
Unamortized debt issuance costs	3,900,000
January 2025 Notes | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	500,000,000																				$ 500,000,000
Outstanding Principal	500,000,000
Net Carrying Value	469,326,000
January 2029 Notes
Debt Instrument [Line Items]
Net Carrying Value	638,778,000																					0
Unamortized debt issuance costs	11,200,000
January 2029 Notes | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	650,000,000																				$ 650,000,000
Outstanding Principal	650,000,000
Net Carrying Value	638,778,000
March 2025 Notes
Debt Instrument [Line Items]
Net Carrying Value	852,378,000																					0
Unamortized debt issuance costs	7,300,000
March 2025 Notes | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	900,000,000															$ 900,000,000
Outstanding Principal	900,000,000
Net Carrying Value	852,378,000
May 2027 Notes
Debt Instrument [Line Items]
Net Carrying Value	593,636,000																					0
Unamortized debt issuance costs	2,500,000
May 2027 Notes | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	625,000,000											$ 625,000,000
Outstanding Principal	625,000,000
Net Carrying Value	593,636,000
April 2026 UK Bonds
Debt Instrument [Line Items]
Net Carrying Value	248,301,000																					0
Unamortized debt issuance costs	3,100,000
April 2026 UK Bonds | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	326,925,000													£ 250,000,000
Outstanding Principal	326,925,000
Net Carrying Value	248,301,000
September 2025 Notes
Debt Instrument [Line Items]
Net Carrying Value	590,541,000																					0
Unamortized debt issuance costs	8,900,000
September 2025 Notes | Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	600,000,000					$ 600,000,000
Outstanding Principal	600,000,000
Net Carrying Value	590,541,000
2021-1 BSL Debt
Debt Instrument [Line Items]
Outstanding Principal	819,500,000
Net Carrying Value	661,975,000																					661,910,000
Unamortized debt issuance costs	1,000,000																					1,100,000
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	661,975,000																					661,910,000
2022-1 BSL Debt
Debt Instrument [Line Items]
Outstanding Principal	589,750,000
Net Carrying Value	418,376,000																					0
Unamortized debt issuance costs	1,600,000
2022-1 BSL Debt | Secured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	420,000,000									$ 589,750,000
Outstanding Principal	531,000,000
Outstanding Principal, Net of Eliminations	420,000,000
Net Carrying Value	418,376,000
2021-2 Debt
Debt Instrument [Line Items]
Outstanding Principal	603,650,000
Net Carrying Value	504,222,000																					504,124,000
Unamortized debt issuance costs	1,600,000																					1,700,000
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	504,222,000																					504,124,000
MML 2021-1 Debt
Debt Instrument [Line Items]
Net Carrying Value	685,943,000																					685,696,000
Unamortized debt issuance costs	4,200,000																					4,300,000
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	685,943,000																					685,696,000
MML 2022-1 Debt
Debt Instrument [Line Items]
Net Carrying Value	753,757,000																					0
Unamortized debt issuance costs	5,300,000
MML 2022-1 Debt | Secured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	759,000,000
Outstanding Principal	759,000,000
Net Carrying Value	753,757,000
MML 2022-2 Debt
Debt Instrument [Line Items]
Outstanding Principal	498,105,000
Net Carrying Value	297,971,000																					0
Unamortized debt issuance costs	2,500,000
MML 2022-2 Debt | Secured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	300,500,000									$ 498,105,000
Outstanding Principal	341,000,000
Outstanding Principal, Net of Eliminations	300,500,000
Net Carrying Value	297,971,000
Short-Term Borrowings
Debt Instrument [Line Items]
Aggregate Principal Committed	844,083,000																					718,156,000
Short-Term Borrowings	$ 844,083,000																					$ 718,156,000