Amounts Recorded for Margin Borrowing Facilities and Subordinated Notes (Parenthetical) (Detail)			1 Months Ended	9 Months Ended	12 Months Ended
	Apr. 25, 2018	Mar. 23, 2017	Aug. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
7.875% Subordinated Notes due 2047
Debt Instrument [Line Items]
Subordinated Notes percentage				7.875%	7.875%
Subordinated Notes due date	2047	2047		2047	2047
7.75% Subordinated Notes due 2045
Debt Instrument [Line Items]
Subordinated Notes percentage			7.75%	7.75%	7.75%
Subordinated Notes due date	2045		2045	2045	2045