Fair Value Measurements (excluding Consolidated Investment Entities) - Significant Unobservable Inputs (Details) - IUL - Investment contract - Market Approach Valuation Technique	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Minimum
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Nonperformance risk	0.20%	0.38%
Lapses	2.00%	2.00%
Mortality	0.00%	0.00%
Maximum
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Nonperformance risk	0.56%	0.84%
Lapses	10.00%	10.00%
Mortality	0.00%	0.00%