Fair Value Measurements (Unobervable Quantitative Input Ratios) (Details) - Recurring - Level 3 - Real estate fund investments	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Minimum
Unobservable Quantitative Input
Discount rates	10.00%	10.00%
Capitalization rate	4.70%	4.30%
Maximum
Unobservable Quantitative Input
Discount rates	14.90%	14.90%
Capitalization rate	5.80%	5.80%
Weighted Average
Unobservable Quantitative Input
Discount rates	12.50%	12.60%
Capitalization rate	5.40%	5.30%