Mortgage Loans - Credit Quality Indicators - MF (Details) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Total mortgage loans	$ 3,902,514,000,000	$ 3,659,247,000,000
Multifamily
Financing Receivable, Credit Quality Indicator [Line Items]
2021	38,603,000,000	72,014,000,000
2020	81,334,000,000	69,337,000,000
2019	67,836,000,000	63,616,000,000
2018	61,015,000,000	53,523,000,000
2017	50,517,000,000	44,753,000,000
Prior	101,936,000,000	74,183,000,000
Total mortgage loans	401,241,000,000	377,426,000,000
Non-classified | Multifamily
Financing Receivable, Credit Quality Indicator [Line Items]
2021	38,547,000,000	71,977,000,000
2020	81,070,000,000	68,296,000,000
2019	66,157,000,000	62,087,000,000
2018	59,144,000,000	50,907,000,000
2017	47,164,000,000	43,174,000,000
Prior	95,395,000,000	70,933,000,000
Total mortgage loans	387,477,000,000	367,374,000,000
Classified | Multifamily
Financing Receivable, Credit Quality Indicator [Line Items]
2021	56,000,000	37,000,000
2020	264,000,000	1,041,000,000
2019	1,679,000,000	1,529,000,000
2018	1,871,000,000	2,616,000,000
2017	3,353,000,000	1,579,000,000
Prior	6,541,000,000	3,250,000,000
Total mortgage loans	13,764,000,000	10,052,000,000
Substandard or doubtful | Multifamily
Financing Receivable, Credit Quality Indicator [Line Items]
Total mortgage loans	$ 0	$ 1,000,000