Fair Value Measurements (Unobervable Quantitative Input Ratios) (Details) - Recurring - Level 3 - Real estate fund investments	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Minimum
Unobservable Quantitative Input
Discount rates	10.00%	2.00%
Capitalization rate	5.10%	4.70%
Maximum
Unobservable Quantitative Input
Discount rates	15.00%	14.90%
Capitalization rate	6.40%	6.70%
Weighted Average
Unobservable Quantitative Input
Discount rates	13.10%	11.90%
Capitalization rate	5.70%	5.50%