Fair Value Measurements (Unobervable Quantitative Input Ratios) (Details) - Level 3 - Recurring - Real estate fund investments	Dec. 31, 2018	Dec. 31, 2017
Measurement Input, Discount Rate | Minimum
Unobservable Quantitative Input
Real estate, measurement input	0.100	0.020
Measurement Input, Discount Rate | Maximum
Unobservable Quantitative Input
Real estate, measurement input	0.150	0.149
Measurement Input, Discount Rate | Weighted Average
Unobservable Quantitative Input
Real estate, measurement input	0.134	0.119
Measurement Input, Cap Rate | Minimum
Unobservable Quantitative Input
Real estate, measurement input	0.054	0.047
Measurement Input, Cap Rate | Maximum
Unobservable Quantitative Input
Real estate, measurement input	0.077	0.067
Measurement Input, Cap Rate | Weighted Average
Unobservable Quantitative Input
Real estate, measurement input	0.057	0.055