Accumulation Units - Schedule of changes in the number of accumulation units outstanding (Details) - shares shares in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Outstanding:
Beginning of period (in shares)	52.0	60.8
Credited for premiums (in shares)	4.7	4.6
Annuity, other periodic payments, withdrawals and death benefits (in shares)	(3.8)	(13.4)
End of period (in shares)	52.9	52.0