Fair Value Measurements (Unobervable Quantitative Input Ratios) (Details) - Recurring - Level 3 - Real estate fund investments	Sep. 30, 2018	Dec. 31, 2017
Discount Rate | Minimum
Unobservable Quantitative Input
Real estate, measurement input	0.100	0.020
Discount Rate | Maximum
Unobservable Quantitative Input
Real estate, measurement input	0.150	0.149
Discount Rate | Weighted Average
Unobservable Quantitative Input
Real estate, measurement input	0.132	0.119
Cap Rate | Minimum
Unobservable Quantitative Input
Real estate, measurement input	0.053	0.047
Cap Rate | Maximum
Unobservable Quantitative Input
Real estate, measurement input	0.064	0.067
Cap Rate | Weighted Average
Unobservable Quantitative Input
Real estate, measurement input	0.056	0.055